freebsd-update: allow user to break out of conflict resolution loop

When a conflict marker is found during update allow the user to confirm
they want it to remain in the file.

Suggested by: Tim Hogard
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Fixes: ceb5f28ba5fc ("freebsd-update: re-edit files if merge confli...")
Differential Revision: https://reviews.freebsd.org/D38896

(cherry picked from commit 3d44241546173de00852ab5e22263ba538cd8759)
(cherry picked from commit 6561fec386a6cbe65b3f9661db797317ab6a99b4)