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/asm/main.c

@@ -298,7 +298,7 @@ main(int argc, char *argv[])
 	if (argc == 1)
 		usage();
 
-	progname = argv[0];
+	progname = "rgbasm";
 
 	/* yydebug=1; */