Detect MVNO using the SPN (bug 1049908)

https://bugzilla.mozilla.org/show_bug.cgi?id=1049908

Tested with a regular DT SIM and a Congstar SIM, it works fine, I get `deutsche_telekom` and `congstar` as a carrier, respectively.
I had to come up with something... I don't like it, if anyone has a better proposal I'll gladly implement it.