This is slightly hacky, part of ussd.c responsible for registering services
is duplicated and parse_ss_control_string is modified to accept a fourth
SI fragment in the input string.
Implement basic SIM filesystem utilities.
Obtain Service Provider Name EF-SPN record.
Make NetworkRegistration use SPN for PLMN display. This implements PLMN
display rules in 31.102 "EFSPN".