* add more comments on units of struct members
* make sure to parsre FC-BVC message correctly
* add error message in case user passes PDU larger than bucket size
* add new function to initialize flow control struct
This code is supposed to implement the BSSGP flow control algorithm,
both for the per-BSS and for the per-MS flow control.
The code currently has no test cases, they will come in a separate
commit.