Use EVFILT_SIGNAL for kqueue latches.

Use EVFILT_SIGNAL for kqueue latches.

Cut down on system calls and other overheads by waiting for SIGURG
explicitly with kqueue instead of using a signal handler and self-pipe.
Affects *BSD and macOS systems.

This leaves only the poll implementation with a signal handler and the
traditional self-pipe trick.

Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com