Merge pull request #155 from Flow-IPC/ipc-148_peercred-misleading

For Channels opened via `ipc::session`, ensure the socket-stream-specific method `remote_peer_process_credentials()` returns values relevant to the opposing process despite the details of how such channels are internally established (namely via `socketpair()`). / Add `Native_socket_stream::remote_peer_process_credentials()` mutator that allows one to correct this value when creating socket-streams more manually (such as is done internally by `ipc::session`).

Author: ygoldfeld