Fix bug in DISCARD-CHAR whereby it would fail if not immediately

Fix bug in DISCARD-CHAR whereby it would fail if not immediately
following a successful (non-EOF) PEEK-CHAR, while it should have the
same effect as READ-CHAR, per the manual.  DISCARD-CHAR is now a port
operation that may be supplied when constructing port types, but for
which a default is provided in terms of READ-CHAR.  The DISCARD-CHAR
feature now clobbers the unread character field only if it is already
filled; otherwise, it defers to the supplied operation.