As part of fixing issue OS#3075, we want to migrate support
for encoding system information from osmo-bsc to libosmocore.
This change ports one of the prerequisites for doing so:
osmo-bsc code for range-encoding ARFCNs, including tests.
An osmo_gsm48_ prefix has been prepended to public symbols in
order to avoid clashes with existing symbols in osmo-bsc code.
Change-Id: Ia220764fba451be5e975ae7c5eefb1a25ac2bf2c
Related: OS#3075
Stefan Sperling