NetworkManager: Fix aliases and dependencies

There are two fixes in this commit.

Firstly, I am creating proper symlinks for the Alias= definitions in the
.service files. This achieves the same result as `systemctl enable`, and
I think is preferred over `mv`.

Secondly, `networkmanager-init` now wants `NetworkManager.service`,
along with `ModemManager.service`. ModemManager does not depend on
NetworkManager (according to `systemctl list-dependencies ModemManager`),
thus NetworkManager never got started on boot.
Merge pull request #1734 from ocharles/network-manager

NetworkManager: Fix aliases and dependencies