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/rpc_generic.c

@@ -122,9 +122,11 @@ __rpc_dtbsize(void)
 	if (tbsize) {
 		return (tbsize);
 	}
+#ifndef __rtems__
 	if (getrlimit(RLIMIT_NOFILE, &rl) == 0) {
 		return (tbsize = (int)rl.rlim_max);
 	}
+#endif /* __rtems__ */
 	/*
 	 * Something wrong.  I'll try to save face by returning a
 	 * pessimistic number.