Babel: Add MAC authentication support - update

Some cleanups and bugfixes to the previous patch, including:

 - Fix rate limiting in index mismatch check

 - Fix missing BABEL_AUTH_INDEX_LEN in auth_tx_overhead computation

 - Fix missing auth_tx_overhead recalculation during reconfiguration

 - Fix pseudoheader construction in babel_auth_sign() (sport vs fport)

 - Fix typecasts for ptrdiffs in log messages

 - Make auth log messages similar to corresponding RIP/OSPF ones

 - Change auth log messages for events that happen during regular
   operation to debug messages

 - Switch meaning of babel_auth_check*() functions for consistency
   with corresponding RIP/OSPF ones

 - Remove requirement for min/max key length, only those required by
   given MAC code are enforced