watchman: enable collection of peer pid on macOS
watchman: enable collection of peer pid on macOS

Summary:
Previously we could capture the peer uid but not the pid.

This copies the same technique that we use in thrift:
`thrift/lib/cpp2/server/Cpp2ConnContext.cpp`

Reviewed By: chadaustin

Differential Revision: D22464126

fbshipit-source-id: aa592e44829a34f7c9a20f8b667eb4b79f28b120

facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman