Refactor 'struct context *ctx = &actx' pattern.

When I'm declaring a local instance of some context structure type to
pass to a function which will pass it in turn to a callback, I've
tended to use a declaration of the form

    struct context actx, *ctx = &actx;

so that the outermost caller can initialise the context, and/or read
out fields of it afterwards, by the same syntax 'ctx->foo' that the
callback function will be using. So you get visual consistency between
the two functions that share this context.

It only just occurred to me that there's a much neater way to declare
a context struct of this kind, which still makes 'ctx' behave like a
pointer in the owning function, and doesn't need all that weird
verbiage or a spare variable name:

    struct context ctx[1];

That's much nicer! I've switched to doing that in all existing cases I
could find, and also in a couple of cases where I hadn't previously
bothered to do the previous more cumbersome idiom.

psftp.c

@@ -1588,7 +1588,7 @@ static bool sftp_action_mv(void *vctx, char *srcfname)
 
 int sftp_cmd_mv(struct sftp_command *cmd)
 {
-    struct sftp_context_mv actx, *ctx = &actx;
+    struct sftp_context_mv ctx[1];
     int i, ret;
 
     if (!backend) {
@@ -1677,7 +1677,7 @@ int sftp_cmd_chmod(struct sftp_command *cmd)
 {
     char *mode;
     int i, ret;
-    struct sftp_context_chmod actx, *ctx = &actx;
+    struct sftp_context_chmod ctx[1];
 
     if (!backend) {
         not_connected();

ssh.c

@@ -1052,21 +1052,21 @@ static const SessionSpecial *ssh_get_specials(Backend *be)
      * and amalgamate the list into one combined one.
      */
 
-    struct ssh_add_special_ctx ctx;
+    struct ssh_add_special_ctx ctx[1];
 
-    ctx.specials = NULL;
+    ctx->specials = NULL;
-    ctx.nspecials = ctx.specials_size = 0;
+    ctx->nspecials = ctx->specials_size = 0;
 
     if (ssh->base_layer)
-        ssh_ppl_get_specials(ssh->base_layer, ssh_add_special, &ctx);
+        ssh_ppl_get_specials(ssh->base_layer, ssh_add_special, ctx);
 
-    if (ctx.specials) {
+    if (ctx->specials) {
         /* If the list is non-empty, terminate it with a SS_EXITMENU. */
-        ssh_add_special(&ctx, NULL, SS_EXITMENU, 0);
+        ssh_add_special(ctx, NULL, SS_EXITMENU, 0);
     }
 
     sfree(ssh->specials);
-    ssh->specials = ctx.specials;
+    ssh->specials = ctx->specials;
     return ssh->specials;
 }
 

tree234.c

@@ -1211,12 +1211,12 @@ int chknode(chkctx * ctx, int level, node234 * node,
 
 void verify(void)
 {
-    chkctx ctx;
+    chkctx ctx[1];
     int i;
     void *p;
 
-    ctx.treedepth = -1;                /* depth unknown yet */
+    ctx->treedepth = -1;                /* depth unknown yet */
-    ctx.elemcount = 0;                 /* no elements seen yet */
+    ctx->elemcount = 0;                 /* no elements seen yet */
     /*
      * Verify validity of tree properties.
      */
@@ -1225,7 +1225,7 @@ void verify(void)
             error("root->parent is %p should be null", tree->root->parent);
         chknode(&ctx, 0, tree->root, NULL, NULL);
     }
-    printf("tree depth: %d\n", ctx.treedepth);
+    printf("tree depth: %d\n", ctx->treedepth);
     /*
      * Enumerate the tree and ensure it matches up to the array.
      */
@@ -1236,17 +1236,17 @@ void verify(void)
             error("enum at position %d: array says %s, tree says %s",
                   i, array[i], p);
     }
-    if (ctx.elemcount != i) {
+    if (ctx->elemcount != i) {
         error("tree really contains %d elements, enum gave %d",
-              ctx.elemcount, i);
+              ctx->elemcount, i);
     }
     if (i < arraylen) {
         error("enum gave only %d elements, array has %d", i, arraylen);
     }
     i = count234(tree);
-    if (ctx.elemcount != i) {
+    if (ctx->elemcount != i) {
         error("tree really contains %d elements, count234 gave %d",
-              ctx.elemcount, i);
+              ctx->elemcount, i);
     }
 }
 

unix/gtkask.c

@@ -569,7 +569,7 @@ const bool buildinfo_gtk_relevant = true;
 char *gtk_askpass_main(const char *display, const char *wintitle,
                        const char *prompt, bool *success)
 {
-    struct askpass_ctx actx, *ctx = &actx;
+    struct askpass_ctx ctx[1];
     const char *err;
 
     ctx->passphrase = NULL;

unix/gtkwin.c

@@ -4408,7 +4408,7 @@ static void compute_geom_hints(GtkFrontend *inst, GdkGeometry *geom)
      * ourselves.
      */
     {
-        struct find_app_menu_bar_ctx actx, *ctx = &actx;
+        struct find_app_menu_bar_ctx ctx[1];
         ctx->area = inst->area;
         ctx->menubar = NULL;
         gtk_container_foreach(GTK_CONTAINER(inst->window),

unix/uxpgnt.c

@@ -498,7 +498,7 @@ void key_find_callback(void *vctx, const char *fingerprint,
 
 struct pageant_pubkey *find_key(const char *string, char **retstr)
 {
-    struct key_find_ctx actx, *ctx = &actx;
+    struct key_find_ctx ctx[1];
     struct pageant_pubkey key_in, *key_ret;
     bool try_file = true, try_fp = true, try_comment = true;
     bool file_errors = false;

windows/winsftp.c

@@ -705,7 +705,7 @@ static DWORD WINAPI command_read_thread(void *param)
 char *ssh_sftp_get_cmdline(const char *prompt, bool no_fds_ok)
 {
     int ret;
-    struct command_read_ctx actx, *ctx = &actx;
+    struct command_read_ctx ctx[1];
     DWORD threadid;
     HANDLE hThread;