MFC r279571,281563 (allanjude):

MFC r279571,281563 (allanjude):

Add a new safetly belt to freebsd-update to prevent a user doing a minor
update (-pX) while having an unfinished major upgrade (9.x to 9.y)
Safetly belt can be disabled with the -F flag

Additionally, add the --not-running-from-cron flag they bypasses the TTY
requirement, and allows freebsd-update to be invoked by orchestration
frameworks, scripts, or otherwise.

PR:		196760
Relnotes:	yes
Sponsored by:	ScaleEngine Inc.

git-svn-id: svn://svn.freebsd.org/base/stable/10@282870 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f