[ifmgmt] Avoid relying on global variable within ifcommon_exec()

The getopt API defines optind as a global variable.  When used by the
"autoboot" command, the payload function passed to ifcommon_exec() may
result in a new iPXE script being executed; the commands therein would
then overwrite the value of optind.  On returning, ifcommon_exec()
would continue processing the list of interfaces from an undefined
point.

Fix by using a local variable to hold the index within the list of
interfaces.

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/commands/ifmgmt_cmd.c

 		    int stop_on_first_success ) {
 	struct ifcommon_options opts;
 	struct net_device *netdev;
+	int i;
 	int rc;
 
 	/* Parse options */
 
 	if ( optind != argc ) {
 		/* Treat arguments as a list of interfaces to try */
-		while ( optind != argc ) {
-			if ( ( rc = parse_netdev ( argv[optind++],
-						   &netdev ) ) != 0 ) {
+		for ( i = optind ; i < argc ; i++ ) {
+			if ( ( rc = parse_netdev ( argv[i], &netdev ) ) != 0 )
 				continue;
-			}
 			if ( ( ( rc = payload ( netdev ) ) == 0 ) &&
 			     stop_on_first_success ) {
 				return 0;