Log Message:
Apply a workaround for NetBSD to the testyield.ml test.

On NetBSD, the "thread scheduler" appears to just schedule two
threads out of 4 when all they in essence to is "yield", and not
do round-robin scheduling, causing the test to loop.  Reduce the
number of threads in the test to 2.

Bump PKGREVISION.

OK'ed by jaapb@