Fix two errors in the conversion to m_mktemp2():

- There was an off-by-one error that prevented the temporary file
  from being created in the same directory as the message it was being
  linked to.
- When using "dist", the assumption was that the temporary file would not
  exist when m_scratch was called (it wanted to link() to the name).  This
  was solved by simply unlink()ing the temporary file after it was created.