freebsd-update: Allow for upper/lowercase y/n

Allow for upper/lowercase y/n in "Does this look right?" question.

Reviewed by: emaste
Approved by: emaste
Differential revision: https://reviews.freebsd.org/D40434

(cherry picked from commit 39f4633b3d2354c01a290d5f422dce7874061769)
(cherry picked from commit a39ddf881ad8530013c5e4cc5f84b3a6b58cca51)

Approved by: re (gjb)