Add CONTINUE restart to blocking calls, and auto-restart them on GC

This patch adds CONTINUE restart to blocking ZMQ calls. To test the CONTINUE restart, do `(setf pzmq:*restart-interrupeted-calls* nil)` then run `(hwserver)` in a different thread, and do `(sb-ext:gc)`, select CONTINUE and server happily continues.

With `*restart-interrupted-calls*` set to T, which is default, calls are automatically restarted on EINTR, other error codes would still offer CONTINUE restart.