posix_mq 0.4.0

Small bugfixes and small API changes to avoid potential
issues/misuse are the focus of this release.

For non-blocking operation, the GVL is no longer bounced.  This
reduces synchronization/scheduling overhead when used in
non-blocking applications.

Small cleanups and documentation improvements, too.

* make POSIX_MQ#dup and POSIX_MQ#clone no-op
* do not release GVL for non-blocking operations
* do not release GVL when unlinking/opening
* POSIX_MQ#<< does not release GVL when non-blocking
* avoid shadow warnings
* README: add mailing list archives info
* POSIX_MQ#to_io works under FreeBSD, too
* fix potential race with notify(&block)
* add TODO item for using netlink under Linux
* remove non-portable #warning CPP directive
* ensure POSIX_MQ#name is clobber-proof