Per-socket multi-thread safety

[m-ronnblom]

Add per-socket multi-thread safety, to allow for example:

1. one receiving and one or more sending threads (both blocking from the threads' point of view) acting on the same XCM socket.
2. allow gracefully terminating a thread waiting (in blocking mode) on xcm_receive() from another thread, by using xcm_close().

The implementation would probably include a POSIX mutex and the use of an XCM socket-internal pipe (or similar) to allow blocking threads to be woken up/notified.

In current XCM, an application that let multiple threads interact with the same socket need to add a wrapper to do roughly the above to work with XCM sockets, or solve this problem in some other way (e.g., to relay messages on an MT safe, process-internal communication channel).