[libc] Restrict access to the RPC Process internals

This patch changes the `Process` struct to only provide the functions
expected to be visible by the interface. So, now we only export the
open, reset, and size query functions. This prevents users of the
interface from messing with the internals of the process, so now the
only existing failure mode is mismatched send and recieve calls.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D150703