Change how filtering routers work.

Commit e7b830e7... was overzealous.  In particular, it took the
`alias' router from working.  Take out `verify = false' from the
`FILTER_TRANSPORT' options, and replace this with a macro
`FILTER_ROUTER' which generates a verify/delivery router pair with the
right stuff in.

This change also introduces verification-time processing for
`~/.mail/forward' and `~/.forward' files, since that's easy and
possibly useful.