Problem: memory leak on message->address

Solution: free correctly in s_message_destroy
Problem: mshell does not reliably send its message

This is due to using dual pipes; the message is still being sent
on the data pipe when the program shuts down with synchronization
on the command pipe. The proper solution is to implement trackers
so mshell can wait for a delivery confirmation before exiting.

Solution: pause for 200msec to give delivery time to complete.

Fixes #35
Problem: receive API is too complex for simple cases

Solution: make _recvx to mirror the _sendx method.
Merge pull request #36 from hintjens/master

Fix mshell send failures, simplify client receive API