Assertion failure in zsockopt.c:965

zsocket_set_linger: Assertion `rc == 0 ... ' failed.

Caused by using zctx_t type for global_context, so that it was trying
to shut down sockets the old way on process exit.

Solution: move global context creation/shutdown to zsys, and use plain
zmq context reference.

Note: this plus new zactor class will result in deprecated classes:
zctx, zmutex, zthread, zsocket, zsockopt.