Fix some borg bugs

* Stop the game crashing when the borg uses msg(NULL), which is an old way of writing message_flush().
* Tell the user where borg.txt should be, because otherwise they might just be really confused like me.
* Fix the borg's logic with identify to bring it in line with gameplay changes over the past few years, while also simplifying the code a lot.
* Fix a bug where the borg would get confused about what inventory screen it was on when identifying (i.e. either inventory or equipment).
* Tell the borg how to dig rubble when it's in exploring mode.
* Make the borg turn off lazy movement delay because it messes up the input routines.
* Get the borg help display working (Ctrl-Z ?)