src: Fix roaming status for MVNOs

Many MVNOs store their equivalent home networks in EF_SPDI. Displayed
name was shown properly, but not the roaming status. This problem does
not affect all modems, as many of them already read SIM files and
report the right status.
src: Fix EF_PNN access

EF_PNN was not being read properly, which affected the displayed PLMN
name for some MVNOs. Some modems already read the file and return the
right string: these do not show the problem.
unit: Fix test to use valid EF_PNN

test-simutil was not using EF_PNN files with the right format.
Merge pull request #52 from alfonsosanchezbeato/mvno-detection

MVNO detection