various fixes for opendkim-db.c

Dear all,

Please find attached a patch for v2.1.0 which fixes the following bugs:
- database (dsn) hostname cannot contain dots (perhaps it was only
tested with "localhost" only by the devs)
- various errors due to missing breaks in DB_TYPE cases

The current statemachine like config string parsing is very error-prone
and hard to debug as for example there are no useful error messages when
the dsn string is unrecognized. One has to actually run a debugger to
figure out where exactly the problem is.

BR,
Jos