Substitute flags from historical mbuf(9) allocator with modern ones.

Sponsored by:	Nginx, Inc.
Remove historical macro.

Sponsored by:	Nginx, Inc.
fix a build problem with INVARIANTS enabled introduced in r260704

Reported by:	glebius
MFC after:	5 days
X-MFC with:	r260704
Another round of removing historical mbuf(9) allocator flags.
They are breeding! New ones arouse since last round.

Sponsored by:	Nginx, Inc.
Simplify wait/nowait code, eventually killing last remnant of
historical mbuf(9) allocator flag.

Sponsored by:	Nginx, Inc.