If a community uses different vpn providers, they typically
assume the same MTU for the wan device underneath the VPN. As
different VPN providers however have different overhead, the MTU
of the VPN device differs for each provider. Therefore this
commit makes the MTU of the VPN device provider specific.

This has two advantages:
1. The same site.conf can used to bake firmwares for different
   VPN providers (only by selecting a diferent vpn feature in the
   site.mk).
2. We are coming closer to the option of integrating multiple VPN
   providers into one firmware.

This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

With the removal of ramips-rt305x, the last user of
GLUON_SPECIALIZE_KERNEL is gone.

This fully abstracts VPN methods, making gluon-mesh-vpn-fastd and
gluon-mesh-vpn-tunneldigger completely self-contained.

Provide a LUA interface for generic interacting with VPN methods in
gluon-mesh-vpn-core and web packages.

This also adds the ability to install tunneldigger and fastd to the same
image, selecting the VPN method based on the selected domain.

Signed-off-by: David Bauer <mail@david-bauer.net>

[Matthias Schiffer: fix up start/stop order and file permissions]

This allows the script to be minified as well as being checked by
luacheck.

The logic is based on the previously used shell script implementation.

Co-Authored-By: Robin Weiligmann <admin@robwei.me>

Signed-off-by: Jan-Tarek Butt <tarek@ring0.de>

In addition to significant internal differences in check_site_lib.lua (in
particular unifying error handling to a single place for the upcoming
multi-domain support), this changes the way fields are addressed in site
check scripts: rather than providing a string like 'next_node.ip6', the
path is passed as an array {'next_node', 'ip6'}.

Other changes in site check scripts:
* need_array and need_table now pass the full path to the sub fields to the
subcheck instead of the key and value
* Any check referring to a field inside a table implies that all higher
levels must be tables if they exist: a check for {'next_node', 'ip6'} adds
an implicit (optional) check for {'next_node'}, which allows to remove many
explicit checks for such tables

Some files have received some additional refactoring.

The generic upgrade script is moved to run after the more specific scripts.
In addition, the script will now remove the configuration sections of
uninstalled VPN packages, so both positive and negative changes of the
default enable state can be migrated correctly.

Based-on-patch-by: Cyrus Fox <cyrus@lambdacore.de>
Fixes: #1187