Fix warnings introduced by ctrl constify
Commit ed9d6da5df98538adc70aa03cb569eb9505d04b6 in libosmocore changes some part of ctrl_cmd struct to const. This is necessary to avoid additional warnings in OpenGGSN ctrl-related code. Regrettably, this results in numerous warnings in OpenBSC du to use of strtok_r for parsing of ctrl_cmd fields.
This should be fixed as follows:
- add unit (or python) tests for functions which have those warnings in OpenBSC
- replace (maybe using some helper functions) direct strtok_r parsing with code working with copies (talloc_strdup?)
- check that tests still pass
- double-check for memleaks