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/usermod.c

@@ -90,7 +90,7 @@
 /*
  * Global variables
  */
-const char *Prog;
+static const char Prog[] = "usermod";
 
 static char *user_name;
 static char *user_newname;
@@ -2158,10 +2158,6 @@ int main (int argc, char **argv)
 #endif				/* USE_PAM */
 #endif				/* ACCT_TOOLS_SETUID */
 
-	/*
-	 * Get my name so that I can use it to report errors.
-	 */
-	Prog = Basename (argv[0]);
 	log_set_progname(Prog);
 	log_set_logfd(stderr);
 
@@ -2172,7 +2168,7 @@ int main (int argc, char **argv)
 	process_root_flag ("-R", argc, argv);
 	prefix = process_prefix_flag ("-P", argc, argv);
 
-	OPENLOG ("usermod");
+	OPENLOG (Prog);
 #ifdef WITH_AUDIT
 	audit_help_open ();
 #endif
@@ -2218,7 +2214,7 @@ int main (int argc, char **argv)
 			exit (1);
 		}
 
-		retval = pam_start ("usermod", pampw->pw_name, &conv, &pamh);
+		retval = pam_start (Prog, pampw->pw_name, &conv, &pamh);
 	}
 
 	if (PAM_SUCCESS == retval) {