DHCP leases for a given interface are already exported on D-Bus
through DHCP4Config and DHCP6Config objects. It is useful to have the
same information also available on the filesystem so that it can be
easily used by scripts.

NM already saves some information about DHCP leases in /var, however
that directory can only be accessed by root, for good reasons.

Append lease options to the existing state file
/run/NetworkManager/devices/$ifindex. Contrary to /var this directory
is not persistent, but it seems more correct to expose the lease only
when it is active and not after it expired or after a reboot.

Since the file is in keyfile format, we add new [dhcp4] and [dhcp6]
sections; however, since some options have the same name for DHCPv4
and DHCPv6, we add a "dhcp4." or "dhcp6." prefix to make the parsing
by scripts (e.g. via "grep") easier.

The option name is the same we use on D-Bus. Since some DHCPv6 options
also have a "dhcp6_" prefix, the key name can contain "dhcp6" twice.

The new sections look like this:

  [dhcp4]
  dhcp4.broadcast_address=172.25.1.255
  dhcp4.dhcp_lease_time=120
  dhcp4.dhcp_server_identifier=172.25.1.4
  dhcp4.domain_name_servers=172.25.1.4
  dhcp4.domain_search=example.com
  dhcp4.expiry=1641214444
  dhcp4.ip_address=172.25.1.182
  dhcp4.next_server=172.25.1.4
  dhcp4.routers=172.25.1.4
  dhcp4.subnet_mask=255.255.255.0

  [dhcp6]
  dhcp6.dhcp6_name_servers=fd01::1
  dhcp6.dhcp6_ntp_servers=ntp.example.com
  dhcp6.ip6_address=fd01::1aa