networkd: optionally change static (and not just transient) hostname based on acquired DHCP lease data

[elschman]

The hostname via option12 should be able to be set static for the host. So that hostname and login screens /etc/hostname and /etc/hosts are adapted if the dhcp server send the hostname.

this was possible in the past with dhcpdclient and exit hooks.

[poettering]

Hmm, yeah, it might make sense to add an option to optionally make DHCP hostname info persistent, if possible, but it definitely shouldn't be the default.

/etc/hosts should never be updated though, it has nothing to do with DHCP.

and "login screens" (you mean gdm and the lot?) display the kernel hostname, so for them this should not be necessary