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

[glebius]

Summary

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

[glebius]

Update with respect to committed changes and snl(3) as well as unstacked
changes to snl(3).

[markj]

lib/libc/rpc/Symbol.map

199

This should go in a new FBSD_1.8 version block, no? See lib/libc/Versions.def.

[glebius]

Address review.

[rmacklem]

lib/libc/rpc/svc_nl.c

89

SVCXPRT *xprt = NULL; so that it can be handled under fail:

126

svc_xprt_free(xprt); ?

[glebius]

lib/libc/rpc/svc_nl.c

126

But there are no failures after svc_xprt_alloc()!

[rmacklem]

lib/libc/rpc/svc_nl.c

126

Yep. It might be a concern if someone adds an
additional failure point after this, but I suppose
it's ok to leave it as is.

[glebius]

Remove the "nl" suffix from the Netlink family name. This is unneeded
tautology. You will see just "rpc" in the genl(1) output. Application
will lookup just "rpc" in the Netlink.

The Linux habit (and our new developing habit) us that Netlink families
are named with what they do, no "nl" prefixes or suffixes, except the
special "netlink control" family which is "nlctrl".