core: loopback - simplify check_loopback()

We no longer configure the addresses on the loopback interface, but simply bring it up
and let the kernel do the rest. Also change the check to only check if the interface
is up, rather than checking for the IPv4 loopback address.
sd-rtnl: recv_message - log when dropping message

We drop messages received from the wrong uid/pid, log this at debug level.
sd-rtnl: recv_message - don't fail on interrupt

We should just try again instead.
sd-rtnl: recv_message - drop message when peeking fails

Read the message form the socket or we will loop trying to read the
same message repeatedly.
sd-rtnl: rtnl_poll - fix typo

This caused rtnl_poll to always return true immediately in sd_rtnl_call().
sd-rtnl: rtnl_call - don't dispatch wqueue after timeout has passed

Only a minor change as the timeout would be hit soon thereafetr at the next loop.
test: loopback - parse logging env var
rtnl: recv_message - don't enforce sender uid

All we care about is that the kernel (pid==0) sent the message. Verifying the sender uid
seems to break when using userns.

Reported by Stéphane Graber.