lib/libc/net/nsdispatch.c: Fix missing unlock and add locking annotations

The error cases (goto fin) of _nsdispatch were missing the unlock.

This change also drops the checks for __isthreaded since the pthread stubs
are already no-ops if threads are not being used. Dropping those conditionals
allows clang's thread safety analysis to deal with the file and also makes
the code a bit more readable. While touching the file also add a few more
assertions in debug mode that the right locks are held.

Reviewed By: markj
Differential Revision: https://reviews.freebsd.org/D29372

(cherry picked from commit 5245bf7b92b74e556527b4916a8deba386fe5772)