Don't use "server:fixed" as the default font without X.

If we're not supporting server-side fonts, it's utterly silly to set
one as the default! Instead, we use Pango's guarantee that some
reasonably sensible monospaced font will be made available under the
name "Monospace", and use that at a reasonable default size of 12pt.

unix/gtkwin.c

@@ -184,7 +184,7 @@ void connection_fatal(void *frontend, const char *p, ...)
 FontSpec *platform_default_fontspec(const char *name)
 {
     if (!strcmp(name, "Font"))
-	return fontspec_new("server:fixed");
+	return fontspec_new(DEFAULT_GTK_FONT);
     else
         return fontspec_new("");
 }