Move gtkwin.c's cmdline_error() into gtkmain.c.

Not all gtkwin-based tools use it. Only the ones with one session per
process, which parse a command line describing that session and might
reasonably want to report errors in that command line by writing to
standard error and exiting the program.

In other words, precisely the ones that link in gtkmain.c and not
gtkapp.c. So gtkmain.c is a more sensible place to put that
error-reporting function.

unix/gtkwin.c

@@ -3789,17 +3789,6 @@ void modalfatalbox(const char *p, ...)
     exit(1);
 }
 
-void cmdline_error(const char *p, ...)
-{
-    va_list ap;
-    fprintf(stderr, "%s: ", appname);
-    va_start(ap, p);
-    vfprintf(stderr, p, ap);
-    va_end(ap);
-    fputc('\n', stderr);
-    exit(1);
-}
-
 const char *get_x_display(void *frontend)
 {
     return gdk_get_display();