This patch doesn't really tests whether osmo-pcu can work on a multi-bts
environment, but it prepares the data structures to be able to do so at
any later point in time.
In Release 5, Fixed Allocations were removed as part of a "GPRS
simplification project" inside 3PGPP. This means that any MS compatible
with Rel-5 or higher may not support it, which makes implementation
in the PCU af historical interest only. See Tdoc GP-021277 for the
actual removal, it happened at 5.0.0 time.