Abe Crabtree complains that flushing the log file as often as we do in 0.56

results in unacceptable performance for him on Win2000. Add a checkbox to
revert to the old behaviour.

[originally from svn r4988]
[this svn revision also touched putty-wishlist]

putty.h

@@ -452,6 +452,7 @@ struct config_tag {
     Filename logfilename;
     int logtype;
     int logxfovr;
+    int logflush;
     int logomitpass;
     int logomitdata;
     int hide_mouseptr;