Add the ability to close sessions. This adds *_free() functions to most

Add the ability to close sessions.  This adds *_free() functions to most
areas of the code.  Not all back-ends have been tested, but Telnet and SSH
behave reasonably.

Incidentally, almost all of this patch was written through Mac PuTTY,
admittedly over a Telnet connection.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2615 cda61777-01e9-0310-a592-d414129be87e