Hardcoding Prog to known value

See #959. We now set Prog (program name) based on hardcoded value instead
of argv[0]. This is to help prevent escape sequence injection.

src/groups.c

@@ -23,7 +23,7 @@
 /*
  * Global variables
  */
-const char *Prog;
+static const char Prog[] = "groups";
 
 /* local function prototypes */
 static void print_groups (const char *member);
@@ -97,10 +97,6 @@ int main (int argc, char **argv)
 	(void) bindtextdomain (PACKAGE, LOCALEDIR);
 	(void) textdomain (PACKAGE);
 
-	/*
-	 * Get the program name so that error messages can use it.
-	 */
-	Prog = Basename (argv[0]);
 	log_set_progname(Prog);
 	log_set_logfd(stderr);