systemd-networkd runs as user "systemd-network" and thus is not privileged to
set the timezone acquired from DHCP:
systemd-networkd[4167]: test_eth42: Could not set timezone: Interactive authentication required.
Similarly to commit e8c0de912, add a polkit rule to grant
org.freedesktop.timedate1.set-timezone to the "systemd-network" system user.
Move the polkit rules from src/hostname/ to src/network/ to avoid too many
small distributed policy snippets (there might be more in the future), as it's
easier to specify the privileges for a particular subject in this case.
Add NetworkdClientTest.test_dhcp_timezone() test case to verify this (for
all people except those in Pacific/Honolulu, there the test doesn't prove
anything -- sorry ☺ ).
