First, we have to use the "keyfile" NetworkManager plugin only; that is, we
disable the "ifupdown" one:

  * it's needed, because the only the "keyfile" plugin supports setting
    dhcp-send-hostname to false, while the "ifupdown" plugin retrieves the
    hostname to send from /etc/hostname;
  * it's OK, because we actually don't use the functionality provided by the
    "ifupdown" plugin (that is, reading from /etc/network/interfaces -- that
    only configures the loopback connection in Tails, which is itself ignored by
    NetworkManager anyway).

Second, we configure the NetworkManager "keyfile" plugin to *not* send the
hostname over DHCP by default. Likely this can be overridden on
a per-connection basis.

Third, we tell dhclient itself not to send the hostname. This is needed because
NetworkManager runs dhclient with the `-cf /var/run/nm-dhclient-eth0.conf`
option, and generates that file by concatenating `/etc/dhcp/dhclient.conf`
with its own settings.