Rename install paths/filenames to unnitrohack

The output program name, data directory and (on *nix) shell script name
are now "unnitrohack" instead of "nitrohack".  This does NOT change the
name of the player config path, library (DLL) names or anything else;
this is just to make it more obvious to the player what they're running.
Add Cygwin build instructions, update last step

Jansson doesn't have a selectable package in Cygwin's setup.exe, so it
has to be compiled and installed by hand here.
Add mintty tips for Cygwin users
Merge branch 'unnethack' of https://github.com/tung/NitroHack into unnethack
Rewrite sort_rooms() to handle lev->doorindex == 0

This replaces the variable length stack arrays and inner function of the
former implementation.  This should hopefully fix level generation for
OS X builds (issue #70).
Merge branch 'unnethack' of https://github.com/tung/NitroHack into unnethack
clean build on OSX