This will make a few good people very happy.  :)

(cherry picked from commit 0d70b4a9)

(cherry picked from commit 234f54d8)

(cherry picked from commit 22ed3586)

(cherry picked from commit 55188651)

(cherry picked from commit 928d169c)

(cherry picked from commit 404caff1)

(cherry picked from commit 204e93a7)

(cherry picked from commit 9fdca300)

(cherry picked from commit 8594ccea)

(cherry picked from commit 23ac5a28)

(cherry picked from commit 14a40cf7)

(cherry picked from commit b6cfa710)

(cherry picked from commit 6d7d0bc3)

(cherry picked from commit 3b42ee7e)

This way, the caller only needs to specify plugins.inc on the
top to let the subsystem(s) know plugin hooks need to be executed.

(cherry picked from commit e8fa56c5)

This is done so that services.inc can load plugins when
plugins.inc was invoked, if not, it stays clear of the
extra work.

(cherry picked from commit 669f6e03)

(cherry picked from commit 269b9b6a)

(cherry picked from commit 4635ac7b)

(cherry picked from commit 40516e54)

(cherry picked from commit 07bafdf6)

(cherry picked from commit 9709336b)

The rules are simple:

* plugins_configure() requires all available plugins, probes their
  configure functions and then calls the advertised functions.

* All GUI pages must directly include their includes in plugins.inc.d
  to not overly pollute their loading phase.

(cherry picked from commit cbd9ffb5)

(cherry picked from commit c9b70766)

(cherry picked from commit 93e47fac)

(cherry picked from commit c68033bc)

(cherry picked from commit 1373b01a)

For now plugins.inc and ipsec.inc will coexist on pages that
may need only one or none to avoid unefined function errors.
This will be pruned after investigating the contents of both
files...

(cherry picked from commit d7df9ecf)

(cherry picked from commit f79fac8f)

(cherry picked from commit 813d76ca)

(cherry picked from commit 7c4bc839)

(cherry picked from commit 7f3e9791)

(cherry picked from commit ff7fce12)

GUI sorting for now should be new, update, reinstall, remove.  Later
all those types ought to be merged into a single list with backwards-
compatible behaviour.  First of many neat firmware improvements on
our way to 16.7.  :)

(cherry picked from commit 8770689a)

This doesn't work out at all for the fact that the control picks
up the first instance it finds, either client or server, but not
clearly stating so.  That's why there is a status page with all
the services for clients and servers...

(cherry picked from commit 7b17ac34)

(cherry picked from commit 918df6ea)

(cherry picked from commit d03b53b3)

fix for https://github.com/opnsense/core/issues/296
and  https://github.com/opnsense/core/issues/347

when no servers are selected, all still apply, which should keep current configurations valid.

(cherry picked from commit d101b28e)

(legacy) align help message with openvpn for vpn_openvpn_server.php, closes https://github.com/opnsense/core/issues/774

(cherry picked from commit 8389079e)

(cherry picked from commit 3f2c5304)

(cherry picked from commit d9c91c3f)