Split rde_filterstate_prep() into three functions.

Split rde_filterstate_prep() into three functions.
- rde_filterstate_init(): initialize a filterstate to default values
- rde_filterstate_copy(): copy from a filterstate into a new state object
- rde_filterstate_prep(): set filtersate based on prefix passed as argument.

This makes the code a bit easier to read.
OK tb@