Add lots of missing 'static' keywords.

A trawl through the code with -Wmissing-prototypes and
-Wmissing-variable-declarations turned up a lot of things that should
have been internal to a particular source file, but were accidentally
global. Keep the namespace clean by making them all static.

(Also, while I'm here, a couple of them were missing a 'const': the
ONE and ZERO arrays in sshcrcda.c, and EMPTY_WINDOW_TITLE in
terminal.c.)

unix/gtkdlg.c

@@ -3279,14 +3279,14 @@ static void messagebox_handler(union control *ctrl, dlgparam *dp,
         dlg_end(dp, ctrl->generic.context.i);
 }
 
-const struct message_box_button button_array_yn[] = {
+static const struct message_box_button button_array_yn[] = {
     {"Yes", 'y', +1, 1},
     {"No", 'n', -1, 0},
 };
 const struct message_box_buttons buttons_yn = {
     button_array_yn, lenof(button_array_yn),
 };
-const struct message_box_button button_array_ok[] = {
+static const struct message_box_button button_array_ok[] = {
     {"OK", 'o', 1, 1},
 };
 const struct message_box_buttons buttons_ok = {

unix/uxpgnt.c

@@ -45,7 +45,7 @@ static void uxpgnt_log(PageantListenerClient *plc, const char *fmt, va_list ap)
     fprintf(pageant_logfp, "\n");
 }
 
-const PageantListenerClientVtable uxpgnt_vtable = {
+static const PageantListenerClientVtable uxpgnt_vtable = {
     uxpgnt_log,
 };
 

unix/x11misc.c

@@ -32,9 +32,8 @@ struct x11_err_to_ignore {
     unsigned long serial;
 };
 
-struct x11_err_to_ignore *errs;
-
-size_t nerrs, errsize;
+static struct x11_err_to_ignore *errs;
+static size_t nerrs, errsize;
 
 static int x11_error_handler(Display *thisdisp, XErrorEvent *err)
 {