show all tractors in the vanilla server

[Trent Piepho <xyzzy@speakeasy.org>]

I've added support for show all tractors into the Vanilla server.  I haven't
committed my changes yet, but unless someone comes up with a reason not to,
I'll do it soon.  It is controlled by SHOW_ALL_TRACTORS in the features file,
and it defaults to OFF.  However, observers will get to have it turned on,
even if it is off in the features file.  This way you can have it off for
normal players, but let observers have it.  When observing, this lets you see
the tractor of the player you are observing, as well as other players.  As it
is now, the player you are observing will appear to always tractor the same
player, normally player 0.

From what I can tell, this feature was first done by Andy McFadden, who also
did the normal show your own tractors on the screen that we all take for
granted now, but I'm sure people screamed borg at the time.  If someone knows
a better history of this feature, I'd love to know about it.  I thought that
it was something created for Paradise, but the comments in the code are tagged
with "ATM", and the code exists, but is not functional, in COW.

COW doesn't support drawing tractor beams for other players, but it does have
code for reading the tractor information from the packets.  It is normally not
turned on; INCLUDE_VISTRACT isn't defined in socket.c.

The Paradise client can display other players' tractors, however it won't do
so when on a bronco server.  I've attached a small patch that should explain
the trivial fix that I did for Paradise 2000, if a paradise worker would like
to make the same change.  This does not allow players with the paradise client
to see other people's tractors on any bronco server, the server god must
explicitly enable the feature.