Since neither I nor Owen know why the IDM_ values for the saved-sessions

Since neither I nor Owen know why the IDM_ values for the saved-sessions
submenu were going up in steps of 16, I've changed to steps of 1, thus
increasing the possible number of sessions from ~256 to 4096, since a
recent report seemed to indicate that the previous limit might not be
enough for someone (!)
I can't find any documentation that puts an upper limit on the number of
menu items, and it seems to work on Win98, which is where I'd expect it to
break if anywhere.
Also a number of other tweaks to this code.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4731 cda61777-01e9-0310-a592-d414129be87e