If a new session was saved from Change Settings, a side-effect on Windows was

If a new session was saved from Change Settings, a side-effect on Windows was
that the global `sesslist' got out of sync with the saved-sessions submenu,
causing the latter to launch the wrong sessions.

Also, Change Settings wasn't getting a fresh session list, so if the set of
sessions had changed since session startup it wouldn't reflect that (at least
until a session was saved). Fixed (on all platforms).

Therefore, since the global sesslist didn't seem to be useful, I've got rid
of it; config.c creates one as needed, as do the frontends. (Not tried
compiling Mac changes.)

Also, we now build the saved-sessions submenu on demand on Windows and Unix.
(This should probably also be done on the Mac.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@5609 cda61777-01e9-0310-a592-d414129be87e