flag a short read on the 2nd file as an EOF when it isn't. This implementation
differs in that at most one read is done per file per comparison, so that an
early mismatch may avoid hanging on input or continuing some expensive process
if reading from a pipe.
EOF message printed to stderr.
Removed bogositys wrt. getopt() processing (special casing `-' is unnecessary).
Updated usage message to document fact that -s and -l options are mutually
exclusive.
