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).

Reviewed by: jhb, kib (earlier version)
MFC after: 20 days
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11680