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.