This function retrieves the tape metadata. See LDSTRDEF.H
 	in the source kit for the layout of the buffer (struct LDTPMD).

 	Function:	IO$_LD_CONTROL

 	P1 = address of a data buffer to retrieve the metadata
 	P2 = buffer length (should be 512 bytes)
 	P6 = Subfunction: LDIO$K_GET_METADATA

 	IOSB word 0 = status

 	Errors returned:

 	SS$_ACCVIO	Buffer can not be written
 	SS$_NODATA	No metadata available
 	SS$_BUFFEROVF	Buffer too small
 	SS$_NORMAL	Successful completion