Export budb_prototypes.h

Export budb_prototypes.h

Other modules need prototypes from the budb directory, so we need
to export this header. However it contains lots of internal functions.
So, split the header into budb_prototypes.h and budb_internal.h, and
reference the internal.h header locally.

Reviewed-on: http://gerrit.openafs.org/44
Verified-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>