[3/3] etcupdate and mergemaster support for certctl

This commit add support for certctl in mergemaster and etcupdate. Both will
either rehash or prompt for rehash as new certificates are
trusted/blacklisted.

This work was done primarily by allanjude@, with minor contributions by
myself.

No objection from: secteam
Differential Revision: https://reviews.freebsd.org/D17389