Add the *_TESTS_SH_SED_* functionality to atf.test.mk.

This exists already in plain.test.mk and tap.test.mk and should have been
added to atf.test.mk too when the feature was first introduced.

(It is probably time to address the related TODOs but I will do that
separately.)
introduce mbq_lock() and mbq_unlock() for the mbq,
so it is easier to buil the same code on linux
(this generalizes the change in svn 267142)

MFC after:	3 days