Summary:
This commit introduces a few types from the watchman codebase:
`FileDescriptor` which is on posix systems represents a file descriptor,
and on Windows is a HANDLE (which can be a file, pipe or socket descriptor).
`Pipe` is a convenience struct that holds the read and write ends of a Pipe.
Note that we have a conceptual class with a windows specific Pipe type under
eden/fs/win/utils/Pipe.h; I remove that in the next diff in the stack.
There are a couple of differences from the watchman code
Reviewed By: chadaustin
Differential Revision: D23287819
fbshipit-source-id: 6ca90ba345037c6c3e308f588d690a899c9866a5
