MSC: void _hardresume( int result )

		MSC: void _hardretn( int error )
		MSC: void _harderr( void (far *handler)() )

	- prototype in dos.h

	- result = return value from handler
	- error = number of error

	- _harderr() registers a user critical error handler with it's
	  own error handler which is to be called during a critical error

	- handler = new INT 24 handler with the format:
	  handler(unsigned deverr,unsigned errcode,unsigned far *devhdr)
	  where: deverr = device error code (AX value DOS passes to INT 24)
		 errcode = error code (DI value DOS passes to INT 21)
		 devhdr = pointer to device header on which error occurred
	- handler must return via one of three methods:
	    simple return  returns to DOS error handler
	    _hardresume()  returns to DOS error handler
	    _hardretn()  returns to the application

	- hardresume() returns one of the following:
	  _HARDERR_ABORT  DOS should abort the process via INT 23
	  _HARDERR_FAIL   DOS should fail the call (DOS 3.x+)
	  _HARDERR_IGNORE DOS should ignore the error
	  _HARDERR_RETRY  DOS should retry the operation
	- due to the complexity of these functions consult the vendor
	  documentation for more specific details

	- Turbo C uses	hardretn(), harderr() and hardresume()
	- see	INT 24