Switch some Conf settings over to being bool.

I think this is the full set of things that ought logically to be
boolean.

One annoyance is that quite a few radio-button controls in config.c
address Conf fields that are now bool rather than int, which means
that the shared handler function can't just access them all with
conf_{get,set}_int. Rather than back out the rigorous separation of
int and bool in conf.c itself, I've just added a similar alternative
handler function for the bool-typed ones.

windows/windefs.c

@@ -29,6 +29,11 @@ char *platform_default_s(const char *name)
     return NULL;
 }
 
+bool platform_default_b(const char *name, bool def)
+{
+    return def;
+}
+
 int platform_default_i(const char *name, int def)
 {
     return def;