openvpn: introduce proto handler

openvpn: introduce proto handler

openvpn needs a proto handler. Here it is.

Removed all of the up/down scripts from the init handler
and made those entirely optional (with some ucode examples).

The config options have been updated to reflect v 2.6/2.7,
with a 'd' flag to denote deprecated. Deprecated flags are
gated behind an 'allow_deprecated' config flag, which must
be on to use them. Some flags will cease to work in the next
version.

Users should not be using compression. Openvpn has enough
security holes and pitfalls already without using
compression.

Updated the example configs (left in place as legacy
documentation) and removed older cryptos which do not exist
in ovpn any longer.

A migration script is included -x. /etc/config/openvpn
entries become interface entries in /etc/config/network
with proto='openvpn'. The source config is retained.

Signed-off-by: Paul Donald <redacted>
https://github.com/openwrt/packages/pull/28533