Add -Wthread-safety to WARNS=6.

While there, add a NO_WTHREAD_SAFETY flag that can be used to disable
this specific warning flag. Disable it for auditdistd. We can easily
patch up auditdistd to have the right annotations to build, but as
auditdistd is intended to be portable across other operating systems,
it's not worth the effort.

Approved by:	brueffer@