libc/rpc: add userland side RPC server over netlink(4)

To be used by NFS related daemons that provide RPC services to the kernel.
Some implementation details inside the new svc_nl.c.

Reviewed by:		rmacklem
Differential Revision:	https://reviews.freebsd.org/D48550

include/rpc/svc.h

@@ -455,6 +455,12 @@ extern SVCXPRT *svc_fd_create(const int, const u_int, const u_int);
  */
 extern SVCXPRT *svcunixfd_create(int, u_int, u_int);
 
+/*
+ * netlink(4) server creation.  To be used to service requests that
+ * originate from an in-kernel client.
+ */
+extern SVCXPRT *svc_nl_create(const char *);
+
 /*
  * Memory based rpc (for speed check and testing)
  */