libmm-glib: add MMLocation3gpp three digit MNC access functions
There are some places in MM where 3-digit MNCs are not very well supported. In the public API this shows in MMLocation3gpp where the MNC is only represented by an integer, so the distinction between e.g. 12301 and 123001 cannot be made.
I added a new function to get the MNC digit status and modified some MNC -> string conversions to always print either 2 or 3 digits.
While working on this, I noticed that some QMI functions lost MNC PCS digit status information and didn't include it in the QMI requests in cases where it would have been needed (e.g. manual registration). I've fixed these as well.
I have not had a chance to test all these in real life, as there are no 3-digit MNCs where I live, so there might still be some issues remaining, but I believe it should anyway work better than before.