libsys: move __libsys_interposer consumers

These system call wrappers call interposed system calls in fairly
trivial ways. Move them over to libsys so all __libsys_interposer
consumers end up in libsys.

Also move recvmmsg and sendmmsg as they are documented with recv and
send.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908