core: use hostnamed to set the transient hostname

core: use hostnamed to set the transient hostname

In commit 6dc35e66d45e ("settings: add hostnamed support") we started
to use systemd-hostnamed for setting the system static hostname
(i.e. the one written to /etc/hostname), but nm-policy.c still called
sethostname() to set the transient (dynamic) hostname when this needs
to be changed, for example after a reverse lookup of our dynamic IP
address.

Thus, when using systemd the hostname change failed because process'
capabilities are restricted and sethostname() requires CAP_SYS_ADMIN.

We should set also the transient hostname through hostnamed when this
is available.

https://bugzilla.redhat.com/show_bug.cgi?id=1308974