SB-SB Docking Bug and Fix

["Michael Moore" <mike@miridea.com>]

There is a bug which allows two starbases to dock.  Silly, I know, but it
happens on base practice servers and could happen in multi-base Chaos,
should someone ever actually play with that setting on...

To reproduce the bug, get two bases on a server.  Call them #1 and #2.  Turn
on #1's docking permission.  Have #2 lock on to #1.  Now have #1 tractor #2.
Voila!  When #2 is dragged close enough, they dock.

The fix is three lines added to orbit.c, and here is the diff:

68a69,73
>    * Two starbases can't dock: */
>   if (me->p_ship.s_type == STARBASE)
>     return FALSE;
>
>   /*

--
Mike