given
		path. "." and ".." are interpreted as usual.
		parameter: a/y=address of path, x maximal length of path
$e7d9	Usedir	Takes a path and applies it to a filename in PCBUF.
		If the filename in PCBUF contains a drive or an absolut
		filename is given, nothing is done. Otherwise the 
		the path is put in front of the filename and is put
		into PCBUF again. The path is not changed. "." and ".."
		are not removed here.
		parameter: a/y=address of path, PCBUF+...
		returns new filename in PCBUF+...
$e7d6	Se