send: Avoid select-specific arguments in {pre,post}_select().

Just pass a pointer to struct sched instead of the fd sets. Since
two of the prototypes declared in send.h now refer to this structure,
sched.h must be included before send.h.

The udp sender implements neither ->pre_select() nor ->post_select(),
so we only need to fix the order in which send.h and sched.h are
included.