rtemsbsd/nfs: Add support to mount NFSv2

- NFSv2 requires userland RPC calls to determine the version of
  NFS and the FH. This is passed to the kernel.

- Port more libc/rpc.

Update #4475

freebsd/lib/libc/rpc/rpcb_clnt.c

@@ -361,7 +361,11 @@ getclnthandle(const char *host, const struct netconfig *nconf, char **targaddr)
 			return (client);
 		}
 	} else {
+#ifndef __rtems__
 		if (getaddrinfo(host, "sunrpc", &hints, &res) != 0) {
+#else /* __rtems__ */
+		if (getaddrinfo(host, "111", &hints, &res) != 0) {
+#endif /* __rtems__ */
 			rpc_createerr.cf_stat = RPC_UNKNOWNHOST;
 			return NULL;
 		}