MFC r337983, r338044:
Add pthread_get_name_np(3).

MFC r303328:

Add links for bit_ffc_at(3) and bit_ffs_at(3).

Approved by: re (kib)

MFC 302899: Add documentation for the sigevent structure.

- Add a sigevent(3) manpage to give a general overview of the sigevent
structure and the available notification mechanisms.
- Document that AIO requests contain a nested sigevent structure that can
be used to request completion notification.
- Expand the sigevent details in other manuals to note details such as
the extra values stored in a queued signal's information or in a posted
kevent.

Approved by: re (gjb)