gluon-wan is a sudo-like exec wrapper that switches the process group to
gluon-mesh-vpn, making it use the WAN dnsmasq rather than resolving over
the mesh.

Note that this only affects DNS at the moment. Processes running under
gluon-wan will still use the regular mesh IPv6 routing table, and not the
WAN routing table. This is not a problem for IPv4, as there is only one
IPv4 routing table.

Fixes #1575

Signed-off-by: Felix Kaechele <felix@kaechele.ca>

This is currently only implemented in the gluon-mesh-vpn-fastd
package.

Advertising the public key may be deemed problematic when
your threat-model involves protecting the nodes privacy
from tunnel traffic correlation by onlink observers.

It can be enabled by setting site.mesh_vpn.fastd.pubkey_privacy
to `false`.

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 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

VPN daemons were always disabled

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

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