Rename 'direction' to 'type' in remote_message_header

Rename 'direction' to 'type' in remote_message_header

The 'remote_message_header' struct has a mis-leadingly named
field 'direction'. It is really a reflection of the type of
message, and some types can be sent in either direction. Thus
the field is more accurately named 'type'. No function change.

* qemud/remote_protocol.x: Rename 'direction' to 'type' in
  'remote_message_header. Write better docs describing the
  message header field semantics & usage
* qemud/remote_protocol.c, qemud/remote_protocol.h: Regenerate
* qemud/remote.c, qemud/dispatch.c, src/remote_internal.c
  Update to reflect rename of 'direction' to 'type'