Implement new network buffer/channel system

The old system dumped an entire packet if a single message was invalid,
the new system has a table of contents so invalid messages can be
skipped.  The new system is slightly more safe as well.

Create a new "packed buffer" (pbuf_t) which essentially is a buf_t that
uses msgpack for serialization.

Remove the "explicit signing" in d_ticcmd.h.

Add several methods to m_buf which were necessary for the new network
buffer/channel system.

Further simplify the main loop; break practically everything out into
separate static functions.

Update error messages in n_proto.c; we're dealing with messages, not
packets here.

Try to be more explicit about char/unsigned char vs. byte; "byte" has
become a sort of terrible plague.

Fix off-by-one bugs when saving player names and messages.

Continue working on sync.