rsyslog: Use %fromhost% instead of %hostname%.

rsyslog: Use %fromhost% instead of %hostname%.

Using hostname did not always work, since that use the hostname
specified in the message itself (In particular some kernel messages
without a hostname were seen). using %fromhost% always uses the hostname
of the host the message was received from. This might break in NAT and
forwarding situations, but those do not apply here.