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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270423 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Merge buffer-printing changes from from projects/ipfw as preparation
for branch merge.

Requested by:	luigi


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270424 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f