Turned the old `Telnet Command' System-submenu into a more general

Turned the old `Telnet Command' System-submenu into a more general
`Special Command' menu, in which any backend can place its own list
of magical things the user might want to ask the backend to do. In
particular I've implemented the recently proposed "break" extension
in SSH2 using this mechanism.
NB this checkin slightly breaks the Mac build, since it needs to
provide at least a stub form of update_specials_menu().

git-svn-id: svn://svn.tartarus.org/sgt/putty@3054 cda61777-01e9-0310-a592-d414129be87e