Restore pre-r239157 handling of sched_yield(), when thread time slice was
aborted, allowing other threads to run.  Without this change thread is just
rescheduled again, that was illustrated by provided test tool.

PR:		192926
Submitted by:	eric@vangyzen.net
MFC after:	2 weeks
Merge buffer-printing changes from from projects/ipfw as preparation
for branch merge.

Requested by:	luigi