THe "null" and "null@l2tp" methods are considered equivalent and always
added and removed together when the method list is "configurable".
"null@l2tp" is added before "null", so it is preferred when the peer
supports both.

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

Fixes: 4249d65a ("treewide: fix luacheck warnings")

- CGI script and index.html are moved from gluon-web to
  gluon-config-mode-core, the script is renamed to 'config'
- gluon-web and gluon-web-model base views and i18n files are symlinked
  into the new path
- gluon-web-theme is renamed to gluon-config-mode-theme and installs
  directly into the new path
- all gluon-web-* models, controllers and views are moved into the new
  path

exec() is moved to gluon.util.

These files are already located in /lib/gluon, no need to add it a second
time.

Pretty much everything about this was broken:
* Fix dependency tracking
* Fix vertical orientation
* Fix paddings
* Add theming

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

[Matthias schiffer: rebase, add a few more restrictions]

Some files have received some additional refactoring.

The fastd_mesh_vpn site.conf section is renamed to mesh_vpn.fastd.