packages: Add some missing dependencies

• at requires cron, since atrun is started by cron and won't work without it, and also requires runtime because at runs jobs under /bin/sh.

• bluetooth requires ppp for rfcomm_pppd (a very common use case).

• bsdconfig and bsdinstall are written in shell script and therefore require /bin/sh.

• devd requires /bin/sh to invoke its hooks.

While here, document the policy for adding dependencies in the README.
This will hopefully ensure we end up with consistent dependencies.

MFC after: 1 day
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D52699