Improve error and warning output

Improve error messages generated by `errx()`, `warnx()` and similar.

Set `progname` to a static string with the name of the program so that
the path of the binary isn't included in error messages.

Signed-off-by: AntonioND <antonio_nd@outlook.com>

src/gfx/main.c

@@ -41,7 +41,7 @@ main(int argc, char *argv[])
 	char *ext;
 	const char *errmsg = "Warning: The PNG's %s setting is not the same as the setting defined on the command line.";
 
-	progname = argv[0];
+	progname = "rgbgfx";
 
 	if (argc == 1) {
 		usage();