pkgbase: fix caroot packaging and add post-install script

The original intention for caroot was to be packaged separately, perhaps so
that users can have a more/less conservative upgrade policy for this
separated from the rest of base.

secure/caroot/Makefile doesn't have anything interesting to package, but its
subdirectories might. Move the PACKAGE= to Makefile.inc so both blacklisted
and trusted get packaged consistently into the correct one rather than the
default -utilities. Also tag the directories for package=caroot, as they
could also be empty; blacklisted is empty by default, but trusted is not.

Add a post-install script to do certctl rehash, along with a note should we
eventually come up with a way to detect that files have been added or
removed that requires a rehash.

-caroot gets a dependency on -utilities, as that's where we provide certctl
at the moment. We can perhaps reconsider this and put certctl into this
package in the future, but there are some bits within -utilities that
unconditionally invoke certctl so let's hold off for now.

Reviewed by: manu (earlier version, before -utilities dep added)
Differential Revision: https://reviews.freebsd.org/D23352