Add a new safetly belt to freebsd-update to prevent a user doing a minor update ...

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
Differential Revision:	https://reviews.freebsd.org/D1550
Reviewed by:	cperciva, delphij
Approved by:	bcr (mentor), rodrigc (src)
MFC after:	1 month
Relnotes:	yes
Sponsored by:	ScaleEngine Inc.