Having laid all the groundwork, we can now remove the global `cfg'

Having laid all the groundwork, we can now remove the global `cfg'
completely from putty.h. It's now static in each of the command-line
front ends, shared only between window.c and windlg.c in PuTTY
proper (I've tested this by doing #define cfg cfgsillyname in those
two files only, and it still links so nobody else is using that
symbol!), and part of the `inst' structure in pterm. I think that
only leaves the Unicode module as the last stubborn holdout in the
anti-global-variables campaign.

[originally from svn r2568]