accept_filter: return different errors for non-listener and a busy socket

accept_filter: return different errors for non-listener and a busy socket

The fact that an accept filter needs to be cleared first before setting to
a different one isn't properly documented.  The requirement that the
socket needs already be listening, although trivial, isn't documented
either.  At least return a more meaningful error than EINVAL for an
existing filter.  Cover this with a test case.