Improvements to SSH1 channel close handling: track sending and

Improvements to SSH1 channel close handling: track sending and
receiving of CLOSE and CLOSE_CONFIRMATION separately rather than
taking short cuts. I believe ssh-1.2.33 sending CLOSE_CONFIRMATION
before CLOSE was causing the remaining incidences of bug
`nonexistent-channel'. (ssh-1.2.33 appears to have unilaterally
decreed that CLOSE and CLOSE_CONFIRMATION are respectively renamed
INPUT_EOF and OUTPUT_CLOSING, hence there is no longer an ordering
constraint on them. Bah.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@1961 cda61777-01e9-0310-a592-d414129be87e