unix socket locking cleanups

[Mark Johnston]

Hi,

I posted a series of patches that try to simplify synchronization in the
unix socket code.  Most of them are bite-sized but D26299 and D26301
require some thought.  The last couple of patches fix some races found
by syzkaller, and an earlier version of the patch series was tested by
Peter Holm.  I'd appreciate any feedback.  Thanks in advance.

https://reviews.freebsd.org/D26294
https://reviews.freebsd.org/D26295
https://reviews.freebsd.org/D26296
https://reviews.freebsd.org/D26297
https://reviews.freebsd.org/D26298
https://reviews.freebsd.org/D26299
https://reviews.freebsd.org/D26300
https://reviews.freebsd.org/D26301