[libc][NFC] Simplifly inbox and outbox state handling

[libc][NFC] Simplifly inbox and outbox state handling

Currently we use a template parameter called `InvertInbox` to invert the
inbox when we load it. This is more easily understood as a static check
on whether or not the process running it is the server. Inverting the
inbox makes the states 1 0 and 0 1 own the buffer, so it's easier to
simply say that the server own the buffer if in != out. Also clean up some of
the comments.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D150365