Split _dbus_fd_set_close_on_exec into Unix and Windows versions

Split _dbus_fd_set_close_on_exec into Unix and Windows versions

On Unix, the thing that can be made close-on-exec is a file descriptor,
which is an int.

On Windows, the thing that can be made close-on-exec is a HANDLE,
which is pointer-sized (but not necessarily a pointer!). In practice,
on Windows we only called _dbus_fd_set_close_on_exec() on socket
pseudo-file-descriptors (SOCKET, which is an unsigned int);
every SOCKET can validly be cast to HANDLE, but not every HANDLE
is a SOCKET.

Before this commit we used an intptr_t as a sort of fake
union { int; HANDLE; }, which just obscures what's going on.

In practice, everything that called _dbus_fd_set_close_on_exec()
is really platform-specific anyway, so let's just have two separate
functions and call this solved.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39610