This makes exportfs fully threaded and solves a problem, posted
by Eric Van Hensbergen:

	% srvfs test /
	% mount /srv/test /n/test
	% lc /n/test/n/test
	[no response]

This was because exportfs did send a Tstat to itself, but since
Tstat was single-threaded, wasn't there a thread to readn() the
message and so it all stucks.