Fix a number of small errors in the port abstraction and its use:

Fix a number of small errors in the port abstraction and its use:

  - Fix WRITE-CHAR methods supplied to MAKE-PORT-TYPE to return the
    number of characters written, i.e. 1, not an unspecific value.
  - Make sure all of the input & output features in the port
    abstraction for transcript ports actually check whether the
    supplied operation succeeded before transcribing the I/O.
  - Use WIDE-STRING-REF, not STRING-REF, on wide strings.
  - Use XSUBSTRING-MOVE!, not SUBSTRING-MOVE!, on external strings.