Port forwarding module now passes backend handles around properly.

As a result I've now been able to turn the global variables `back'
and `backhandle' into module-level statics in the individual front
ends. Now _that's_ progress!

[originally from svn r2142]

plink.c

@@ -69,6 +69,9 @@ DWORD orig_console_mode;
 
 WSAEVENT netevent;
 
+static Backend *back;
+static void *backhandle;
+
 int term_ldisc(Terminal *term, int mode)
 {
     return FALSE;