Implement SIGEV_THREAD notifications for lio_listio(2)

[asomers]

Summary

Implement SIGEV_THREAD notifications for lio_listio(2)

Our man pages have always indicated that this was supported, but in fact the
feature was never implemented for lio_listio(2).

Test Plan

tests/sys/aio/lio_test

[jhb]

This generally looks fine. The saved_ev thing is kind of racey (what if the thread fires before the sigevent contents are updated?), but that isn't a new bug.

lib/librt/aio.c

214 ↗

(On Diff #31028)

'sig == NULL' instead of '!sig'