Initialize file descriptors to ignore.

Poll ignores file descriptors passed with -1, so all peer slots not filled
should be set to -1.
Change file descriptor array offset to 1, and clean up after peers.

The file descriptor array offset was previously two, because aside from
peers, each node had a udp socket, and a master tcp socket. Now there is
only a master tcp socket preceding the sockets for peers.

After a peer is done with a slot, the file descriptor should be set back to
-1 so it is ignored in poll.
Fix reference to peer\'s position.
Compile with debugging symbols.
Remove address data and give mikpeer correct function prototypes.
Implement sending and receiving messages.

Miknet can now send and receive messages. mikpeer_send will
queue a packet to be sent, and it will send when the miknode is allowed
to operate (poll).

Some small fixes are also included, such as a typo left over offsetting the
fds array by 2 instead of 1. Nothing that should change the intended
behavior or rationale behind those modules.

The test code is updated for convenience.