[dhcp] Allow multiple interfaces in dhcp command

The "dhcp" command now accepts a list of interfaces to try until one
succeeds.  For example:

 iPXE> dhcp net0 net1 net2

If no interfaces are specified, all interfaces will be tried.

Note that interfaces that fail to DHCP are closed in order to avoid
memory exhaustion.  This behavior differs from the previous "dhcp"
command implementation but should not affect any existing scripts
since a "dhcp" command failure would in any case cause the script to
abort.

Originally-implemented-by: Lars Kellogg-Stedman <lars@oddbit.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/commands/dhcp_cmd.c

  */
 static void dhcp_syntax ( char **argv ) {
 	printf ( "Usage:\n"
-		 "  %s <interface>\n"
+		 "  %s [<interface>] [<interface>...]\n"
 		 "\n"
 		 "Configure a network interface using DHCP\n",
 		 argv[0] );
 }
 
+/**
+ * Execute "dhcp" command for a network device
+ *
+ * @v netdev		Network device
+ * @ret rc		Exit code
+ */
+static int dhcp_exec_netdev ( struct net_device *netdev ) {
+	int rc;
+
+	if ( ( rc = dhcp ( netdev ) ) != 0 ) {
+		printf ( "Could not configure %s: %s\n",
+			 netdev->name, strerror ( rc ) );
+
+		/* Close device on failure, to avoid memory exhaustion */
+		netdev_close ( netdev );
+
+		return 1;
+	}
+
+	return 0;
+}
+
+/**
+ * Execute "dhcp" command for a named network device
+ *
+ * @v netdev_name	Network device name
+ * @ret rc		Exit code
+ */
+static int dhcp_exec_name ( const char *netdev_name ) {
+	struct net_device *netdev;
+
+	netdev = find_netdev ( netdev_name );
+	if ( ! netdev ) {
+		printf ( "No such interface \"%s\"\n", netdev_name );
+		return 1;
+	}
+
+	return dhcp_exec_netdev ( netdev );
+}
+
 /**
  * The "dhcp" command
  *
 		{ "help", 0, NULL, 'h' },
 		{ NULL, 0, NULL, 0 },
 	};
-	const char *netdev_txt;
+	const char *netdev_name;
 	struct net_device *netdev;
 	int c;
 	int rc;
 		}
 	}
 
-	/* Need exactly one interface name remaining after the options */
-	if ( optind != ( argc - 1 ) ) {
-		dhcp_syntax ( argv );
-		return 1;
-	}
-	netdev_txt = argv[optind];
-
-	/* Parse arguments */
-	netdev = find_netdev ( netdev_txt );
-	if ( ! netdev ) {
-		printf ( "No such interface: %s\n", netdev_txt );
-		return 1;
-	}
-
-	/* Perform DHCP */
-	if ( ( rc = dhcp ( netdev ) ) != 0 ) {
-		printf ( "Could not configure %s: %s\n", netdev->name,
-			 strerror ( rc ) );
-		return 1;
+	if ( optind != argc ) {
+		/* Treat arguments as a list of interfaces to try */
+		while ( optind != argc ) {
+			netdev_name = argv[optind++];
+			if ( ( rc = dhcp_exec_name ( netdev_name ) ) == 0 )
+				return 0;
+		}
+	} else {
+		/* Try all interfaces */
+		for_each_netdev ( netdev ) {
+			if ( ( rc = dhcp_exec_netdev ( netdev ) ) == 0 )
+				return 0;
+		}
 	}
 
-	return 0;
+	return 1;
 }
 
 /**