(legacy) first step in transition of /themes/ to /ui/themes/  to glue both new and old components better together.
The old /themes/ directory should eventually be removed when all references are gone.

As an added benefit, (default) will now always revert back to the
actual default from the sample configuration, which is exactly
how it should be.

o Makes live-mounting easier as the package config samples won't
  be clobbered anymore.

o Automatically picks up ".in" files, runs replace and moves them
  to their permanent location.

o Refactored src directory install while at it.

Services -> DHCP Server

This reverts commit 92a5f77a.
This reverts commit 681f654c.

for example, if squid needs some directories to exists before initial run after boot, it could set this in /etc/rc.d.conf/squid

squid_opnsense_bootup_run="/usr/local/opnsense/scripts/proxy/setup.sh"

which effectively results in setup.sh being started followed by the normal service squid start

disadvantage of this, both the service template and the rc system needed to be aware of the setup.
advantage, all services which don't need special preperation are configured as they would be on normal FreeBSD