[libc] Remove 'try_open' from the client interface

We previously provided the `try_open` facility to indicate if opening a
port failed. This is used on the server to continuously poll and quit if
there is no work. However the Client currently has no way to recover
from not finding a port and simply spins repeatedly. The abstraction
here costs us some resources. This patch changes the interface to only
allow `open` on the client side and merges the loops. This saves us a
branch and a good number of registers.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D158365