Posix_mq 2.0.0 - POSIX Message Queues for Ruby

POSIX message queues allow local processes to exchange data in the form
of messages. This API is distinct from that provided by System V
message queues, but provides similar functionality.

Changes: a minor API change + FreeBSD fixes

Blocking methods no longer raise Errno::EINTR (to match
Ruby IO semantics, making code easier to work with).

There are also many FreeBSD-related fixes thanks to
Hleb V. [email protected] and small code cleanups.