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