Move rpc and netconfig to lib/libc/rpc/

This uses relative paths to make it more specific to avoid any potential
future problems with .PATH and leverages CONFS.

libc was picked as the destination location for these because of the syscalls
that use these files as the lowest level place they are referenced.

Approved by: re (gjb), will (mentor)
Differential Revision: https://reviews.freebsd.org/D17163