The Mac OS X 10.5 SDK requires the second argument of FD_ISSET to be
 writeable, although it does only access the data. Given that we have a
 const pointer for a const struct, copy and pass that.
 .
 Note that this is merely a workaround for OS X 10.5, as 10.6 and later
 define the second argument of FD_ISSET as const struct const *foo, too.
 .
 It is safe, as data is accessed read-only by FD_ISSET, even on 10.5.