Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

More-accurately replicate the help system from sysinstall(8). However, also
improve upon the system by giving the user a "Help" button instead of requiring
the user to press F1.

NOTE: In FreeBSD-9 and higher, dialog(1) does not support the F1 hook, so the
mechanism for providing help to the user had to be changed to a button.

This now means we can resurrect *.hlp files from usr.sbin/sysinstall/help/ as-
is and reusing them as-needed in bsdconfig (holding to the goal of losing as
little functionality from sysinstall as possible).

Reviewed by: adrian (co-mentor)
Approved by: adrian (co-mentor)