Update unix domain socket locking comments.

• Define a locking key for unpcb members.
• Rewrite some of the locking protocol description to make it less verbose and avoid referencing some subroutines which will be renamed.
• Reorder includes.

Reviewed by: glebius, kevans, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26294