lib/libc/tests/rpc: Correctly set timeout

The rpc_control() API does not accept the CLCR_SET_RPCB_TIMEOUT command,
it only accepts RPC_SVC_CONNMAXREC_GET/RPC_SVC_CONNMAXREC_SET, so it was
not doing anything.
Instead of incorrectly calling this API, use clnt_create_timed() instead.

I noticed this because the test was timing out after 120s in the CheriBSD CI.

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D28478

Remove expected failure now that it was fixed in r317660.

PR: 211804
Reviewed by: ngie
Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10576

Diff reduce with upstream by using macros and logic integrated in
later revisions of the test

Fix lib/libc/rpc test assumptions added in r305358

- Require root in the tcp/udp subtests (it's needed on FreeBSD when
registering services).
- Skip the tests if service registration fails.

MFC after: 59 days
X-MFC with: r305358
Reported by: Jenkins, rodrigc
Sponsored by: EMC / Isilon Storage Division

Expect :raw to fail on FreeBSD

clnt_raw fails with `RPC_CANTDECODERES` today with the testcase
provided by NetBSD.

PR: 211804
Sponsored by: EMC / Isilon Storage Division

Cast xdr_void to xdrproc_t to mute -Wincompatible-pointer-types warnings from
clang

This pattern is used in other areas of lib/libc/rpc

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

rpc_control on FreeBSD is a public-ish API (not prefixed with __), not private
like NetBSD

Submitted by: pho