Cygwin debugging enhancements

[Christopher Faylor]

I added two things to cygwin to help with debugging the DLL itself.

The output from CYGWIN_SLEEP now includes the program name.

I sometimes set CYGWIN_SLEEP to zero just to see what is starting
and this helps figure out where crashes occur.

I also had the dll honor CYGWIN_DEBUG so that if you

set CYGWIN_DEBUG=cat

gdb.exe will be started when the cat program is detected.

I've documented this in how-to-debug-cygwin.txt.

FYI,
cgf