MFC r270992:
Fix logical error.

Approved by:	re, glebius
MFC r271017 and r271018:
Add new quirk.

PR:        193279
Approved by:	re, marius
MFC r271000:
Delay the return from thread_single(SINGLE_EXIT) until all threads are
really destroyed by thread_stash() after the last switch out.

MFC r271007:
Retire thread_unthread().

MFC r271008:
Style.

Approved by:	re (marius)