hostnamed: allow networkd to set the transient hostname

hostnamed: allow networkd to set the transient hostname

systemd-networkd runs as user "systemd-network" and thus is not privileged to
set the transient hostname:

  systemd-networkd[516]: ens3: Could not set hostname: Interactive authentication required.

Standard polkit *.policy files do not have a syntax for granting privileges to
a user, so ship a pklocalauthority (for polkit < 106) and a JavaScript rules
file (for polkit >= 106) that grants the "systemd-network" system user that
privilege.

Add DnsmasqClientTest.test_transient_hostname() test to networkd-test.py to
cover this. Make do_test() a bit more flexible by interpreting "coldplug==None"
as "test sets up the interface by itself". Change DnsmasqClientTest to set up
test_eth42 with a fixed MAC address so that we can configure dnsmasq to send a
special host name for that.

Fixes #4646