Various entropy integration improvements.

- New /etc/security check for entropy in daily security report.

- New /etc/rc.d/entropy script runs (after random_seed and rndctl) to
check for entropy at boot -- in rc.conf, you can:

. set `entropy=check' to halt multiuser boot and enter single-user
mode if not enough entropy

. set `entropy=wait' to make multiuser boot wait until enough entropy

Default is to always boot without waiting -- and rely on other
channels like security report to alert the operator if there's a
problem.

- New man page entropy(7) discussing the higher-level concepts and
system integration with cross-references.

- New paragraph in afterboot(8) about entropy citing entropy(7) for
more details.

This change addresses many of the issues discussed in security/55659.
This is a first draft; happy to take improvements to the man pages and
scripted messages to improve clarity.

I considered changing motd to include an entropy warning with a
reference to the entropy(7) man page, but it's a little trickier:
- Not sure it's appropriate for all users to see at login rather than
users who have power to affect the entropy estimate (maybe it is,
just haven't decided).
- We only have a mechanism for changing once at boot; the message would
remain until next boot even if an operator adds enough entropy.
- The mechanism isn't really conducive to making a message appear
conditionally from boot to boot.