MTIO mods:

    (1)	Install error checking in MTOPEN ("errchk open").

    (2) Attempt to position to a file beyond EOT for reading should not
	cause an error, rather EOF should be returned at the first read,
	indicating a zero length file (i.e., EOT).

    (3) ZARDMT should zero-fill to an integral number of chars, provided
	space is available at end of buffer (see ZARDBF, which I had to
	modify to provide zero-fill).