Giant const-correctness patch of doom!

Having found a lot of unfixed constness issues in recent development,
I thought perhaps it was time to get proactive, so I compiled the
whole codebase with -Wwrite-strings. That turned up a huge load of
const problems, which I've fixed in this commit: the Unix build now
goes cleanly through with -Wwrite-strings, and the Windows build is as
close as I could get it (there are some lingering issues due to
occasional Windows API functions like AcquireCredentialsHandle not
having the right constness).

Notable fallout beyond the purely mechanical changing of types:
 - the stuff saved by cmdline_save_param() is now explicitly
   dupstr()ed, and freed in cmdline_run_saved.
 - I couldn't make both string arguments to cmdline_process_param()
   const, because it intentionally writes to one of them in the case
   where it's the argument to -pw (in the vain hope of being at least
   slightly friendly to 'ps'), so elsewhere I had to temporarily
   dupstr() something for the sake of passing it to that function
 - I had to invent a silly parallel version of const_cmp() so I could
   pass const string literals in to lookup functions.
 - stripslashes() in pscp.c and psftp.c has the annoying strchr nature

macosx/osxctrls.m

@@ -1736,7 +1736,7 @@ void dlg_beep(void *dv)
     NSBeep();
 }
 
-void dlg_error_msg(void *dv, char *msg)
+void dlg_error_msg(void *dv, const char *msg)
 {
     /* FIXME */
 }

macosx/osxdlg.m

@@ -490,7 +490,7 @@ static void connection_fatal_callback(void *ctx, int result)
     [win endSession:FALSE];
 }
 
-void connection_fatal(void *frontend, char *p, ...)
+void connection_fatal(void *frontend, const char *p, ...)
 {
     SessionWindow *win = (SessionWindow *)frontend;
     va_list ap;

macosx/osxmain.m

@@ -58,7 +58,7 @@ char *x_get_default(const char *key)
     return NULL;		       /* this is a stub */
 }
 
-static void commonfatalbox(char *p, va_list ap)
+static void commonfatalbox(const char *p, va_list ap)
 {
     char errorbuf[2048];
     NSAlert *alert;
@@ -85,7 +85,7 @@ static void commonfatalbox(char *p, va_list ap)
     exit(1);
 }
 
-void nonfatal(void *frontend, char *p, ...)
+void nonfatal(void *frontend, const char *p, ...)
 {
     char *errorbuf;
     NSAlert *alert;
@@ -103,7 +103,7 @@ void nonfatal(void *frontend, char *p, ...)
     sfree(errorbuf);
 }
 
-void fatalbox(char *p, ...)
+void fatalbox(const char *p, ...)
 {
     va_list ap;
     va_start(ap, p);
@@ -111,7 +111,7 @@ void fatalbox(char *p, ...)
     va_end(ap);
 }
 
-void modalfatalbox(char *p, ...)
+void modalfatalbox(const char *p, ...)
 {
     va_list ap;
     va_start(ap, p);
@@ -119,7 +119,7 @@ void modalfatalbox(char *p, ...)
     va_end(ap);
 }
 
-void cmdline_error(char *p, ...)
+void cmdline_error(const char *p, ...)
 {
     va_list ap;
     fprintf(stderr, "%s: ", appname);

macosx/osxwin.m

@@ -888,7 +888,7 @@ int from_backend_untrusted(void *frontend, const char *data, int len)
     return [win fromBackendUntrusted:data len:len];
 }
 
-int get_userpass_input(prompts_t *p, unsigned char *in, int inlen)
+int get_userpass_input(prompts_t *p, const unsigned char *in, int inlen)
 {
     SessionWindow *win = (SessionWindow *)p->frontend;
     Terminal *term = [win term];