Add a socket destructor callback. This allows kernel providers to set
callbacks to perform additional cleanup actions at the time a socket is
closed.

Michio Honda presented a use for this at BSDCan 2018.
(See https://www.bsdcan.org/2018/schedule/events/965.en.html .)

Submitted by: Michio Honda <micchie at sfc.wide.ad.jp> (previous version)
Reviewed by: lstewart (previous version)
Differential Revision: https://reviews.freebsd.org/D15706