Fix cut-and-paste errors in nonfatal() implementations.

Unix GUI programs should not say 'Fatal Error' in the message box
title, and Plink should not destroy its logging context as a side
effect of printing a non-fatal error. Both appear to have been due to
inattentive cut and paste from the pre-existing fatal error functions.

[originally from svn r10044]

unix/gtkdlg.c

@@ -3415,7 +3415,7 @@ void nonfatal(char *p, ...)
     va_start(ap, p);
     msg = dupvprintf(p, ap);
     va_end(ap);
-    fatal_message_box(NULL, msg);
+    nonfatal_message_box(NULL, msg);
     sfree(msg);
 }