systemd: pull in network.service and introduce NetworkManager-wait-online.service

systemd: pull in network.service and introduce NetworkManager-wait-online.service

This pulls in network.target from NetworkManager.service (and not the
other way round), as suggested and agreed on on the systemd ML:

http://lists.freedesktop.org/archives/systemd-devel/2011-March/001692.html

This also introduces an auxiliary service
NetworkManager-wait-online.service that can be used to order a unit
after the point where the network is available. When this is enabled
with "systemd enable NetworkManager-wait-online.service" the unit
network.target will be delayed until the network is up, which is
suitable for synchronizing NFS mounts and similar to it.

https://bugzilla.redhat.com/show_bug.cgi?id=692008