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/fix/main.c

@@ -71,7 +71,7 @@ main(int argc, char *argv[])
 	int version;   /* mask ROM version number */
 	int padvalue;  /* to pad the rom with if it changes size */
 
-	progname = argv[0];
+	progname = "rgbfix";
 
 	while ((ch = getopt(argc, argv, "Cci:jk:l:m:n:p:sr:t:v")) != -1) {
 		switch (ch) {