recv: Make ->btrp and ->fd generic.

recv: Make ->btrp and ->fd generic.

All three receivers maintain a file descriptor and a buffer tree
pool per connection. Currently these are part of the receiver node's
private_data structure, which is a needless code duplication.

This patch moves both fields to the generic struct receiver_node. Since
the private_data structure of the udp and dccp receivers contained
no other fields, this allows to get rid of it completely for these
two receivers.