Replace va_list fmt() overload with vfmt()

Using an overload that takes a va_list argument potentially causes
accidental misuse on platforms (e.g. 32-bit) where va_list is
implemented as a type that may collide with commonly-used argument
types.

For example:

    char* c = copy_string("hi");
    fmt("%s", (const char*)c);
    fmt("%s", c);

The first fmt() call correctly goes through fmt(const char*, ...) first,
but the second mistakenly goes through fmt(const char*, va_list) first
because variadic function overloads have lower priority during overload
resolution and va_list on a 32-bit system happens to be defined as a
pointer type that can match with "char*" but not "const char*".

src/broker/Manager.cc

@@ -625,7 +625,7 @@ void Manager::Error(const char* format, ...)
 	{
 	va_list args;
 	va_start(args, format);
-	auto msg = fmt(format, args);
+	auto msg = vfmt(format, args);
 	va_end(args);
 
 	if ( script_scope )