libathemecore/flags: Add flag 'f' for MC_ANTIFLOOD to mc_flags.

This will allow the ANTIFLOOD flag to be saved to and loaded from the storage
backend.  This resolves #333.
Merge pull request #334 from lstarnes1024/master+antifloodfix

libathemecore/flags: Add flag 'f' for MC_ANTIFLOOD to mc_flags (fix bug #333)