bsdinstall: do a certctl rehash upon installation of configuration

If certctl is installed on the system we're configuring, do a certctl
rehash.

Note that certctl may not be present if the world we've installed was built
either WITHOUT_OPENSSL or WITHOUT_CAROOT. In this scenario, we don't
currently see if the host has a certctl as this may be an indication that
the system *shouldn't* have certs installed into /etc/ssl.

Reviewed by: allanjude, dteske
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D24640