(openvpn client export) add X-Content-Type-Options: nosniff header, closes https://github.com/opnsense/core/issues/1204

Uh, oh: the keys are not purged on factory reset.  Since we
now store the explicit host key file names in the config,
it's better to stop writing keys to /usr/local/etc/ssh and
just keep the backup where it was.  It's fully compatible
with our approach.

While there, remove the need to softcode commands in variables
as this was done to migrate from base OpenSSH to ports OpenSSH,
which was done in 2015.

I pulled this from the attic, but it makes no sense.  ;)

The rules edit page can do it, but it was not picked up
by the actual firewall rules...

Revert "(sshd) add HostKey to sshd_config to force available hostkeys. closes https://github.com/opnsense/core/issues/1200"

This reverts commit 4e83616c.

We need more input from the reporter, nobody has ran into this error
as far as I could see, testing was ok, and when OpenSSH switches
to the next version this will automatically align itself...

Fix a few style errors while there.

(sshd) add HostKey to sshd_config to force available hostkeys. closes https://github.com/opnsense/core/issues/1200

Now that we have a plist it's easier to review/preview changes.
Can finally tackle the src/opnsense/contrib mess in a sensible
way with this and since we do not rely on mount/umount anymore.

When we build packages, we want to know if something is wrong.
For now, only use it in the upgrade path, not sure how to
to distinguish if a user or the build system is running the
package build.

Transition away from making the internals known, so rework
the whole tree.mk to expand better to dynamic destinations.

This is not hooked up yet, but we're more and more progressing
towards a package on top of FreeBSD.  The loader changes are
now handled via rc.loader.d, it makes sense to pll these two
files in as well, obsoleting the need to play tricks with src.git.

The file came from Scott Dale in 2007 like some of the others.
While there, also fix Deciso update from 2014->2016 in our tree.

See: https://github.com/pfsense/pfsense/commit/ab6a5cfc