MSC: unsigned _dos_open( char *fname, unsigned mode, int *handle )

	- prototype in dos.h, fcntl.h (also include share.h)

	- fname = valid DOS file name
	- mode	= file access permission (a logical or of the following)
		= O_NOINHERIT	file can't be inherited by child process
		= O_RDONLY	file is read only
		= O_RDWR	file is read/write
		= O_WRONLY	file is write only
		= SH_COMPAT	file sharing compatibility mode
		= SH_DENYNONE	file sharing deny neither read nor write
		= SH_DENYRD	file sharing deny read
		= SH_DENYRW	file sharing deny read/write
		= SH_DENYWR	file sharing deny write

	- handle = pointer to memory to contain handle
	- does not creat files, they must currently exist (see _dos_creat())
	- returns 0 on success or DOS error code

	- Turbo C uses _open()
	- see	INT 21,3D  _dos_close()  _dos_read()
		_dos_write()	_dos_creat()