Compatibility: 3.0+
Expects: AH    5aH
DS:DX address of ASCIIZ  path (i.e., d:\path\0)
                                                                  
Returns: AX    error code if CF is set to CY
file handle (if no error)
DS:DX (+n) filename has been appended to path
                                                                  
Info: Creates (opens) a file with a unique filename in a specified
directory and returns a handle to that file and its full name.
The DOS command processor uses this function to create the
temporary "piping" files used in I/O redirection.

The pathspec must be ready to receive a filename at its end; that
is, the last character must be a backslash (\) and you must
provide at least 12 bytes at the end of the string.  In summary,
it must be in the form:

"d:\path\",0  (specify drive and path)       ...OR...
"d:",0        (default directory of a drive) ...OR...
"d:\",0       (root directory of a drive)    ...OR...
"",0          (default drive and directory)

Upon return, the string at DS:DX has a unique filename appended
to it and the file is open for read/write Access Mode.

Notes:   DOS builds a filename of hex digits obtained from the current
system date/time.  If the filename already exists, DOS keeps
trying new names until a file can be created.

  The files created ARE NOT really TEMPORARY, and must be deleted
via Fn 41H if you don't want them to clutter up your
directories.

  It is good practice to check the environment for a variable
named TEMP and use its value as the directory for the file.

- -

DOS Fn 5aH: Create Unique Temporary File