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 also drops the GMAC-based methods from gluon-mesh-vpn-fastd's
check_site.lua, as they are not supported anymore.

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]

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

The Gluon-specific functions of the gluon-simple-tc package are moved to
gluon-mesh-vpn-fastd.

This patch allows fastd's enabled flag's default value to be set from
site.conf.