This commit introduces two additional fastd site.conf options:
`sysupgrade_remove_old_peers` removes peer and peer group entries
existing from an old configuration except those which were
defined using the node2node VPN feature after a sysupgrade.
`sysupgrade_remove_n2n_peers` removes existing peer and peer group
entries which were defined using the node2node VPN feature
(which have 'n2n_vpn' in their name) after a sysupgrade.
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
