move firmware mirror/flavour settings to firmware page, closes https://github.com/opnsense/core/issues/834

first step in moving firmware settings to firmware page, https://github.com/opnsense/core/issues/834

If one deletes "plugins.inc", this way the plugin system can be
disabled.  That's a lot better than removing plugins.inc.d/
content and executing the stubs everytime anyway.

Clean-cut move to individual plugins.  Ad finished his interface
plugin rework so now it's time.  :)

As before, make this a mandatory feature for dhcp chroot in order
to catch problems with the implemenation as early as possible.

some cleanup and translation

Add a test case where it hurts: syslog startup.  :)

(interfaces.inc) cleanup interface_proxyarp_configure and actually use provided pid, closes https://github.com/opnsense/core/issues/981

o Allow side-stepping flock(1) if not installed for rc script
o Pass arguments to subroutine script to allow passing modes
o Print a busy message when e.g. the GUI is running the upgrade

We do not want to have rc.firmware use the other scripts for
a number of reasons...

* Don't jump through configd in case it's dysfunct
* Don't use the normal script in case it's gone
* tee(1) output so that the console works as usual