Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
POSIX requires sigevent to be visible after mqueue.h is included.
Add function __mq_oshandle() to get file handle of a mqueue.
Fix compiling for c++, include cdefs.h.
Bring in POSIX message queue header file.