_dbus_read_socket_with_unix_fds: do not accept extra fds in cmsg padding

_dbus_read_socket_with_unix_fds: do not accept extra fds in cmsg padding

This addresses CVE-2014-3635.

If (*n_fds * sizeof (int) % sizeof (size_t)) is nonzero,
then CMSG_SPACE (*n_fds * sizeof (int)) > CMSG_LEN (*n_fds * sizeof (int)
because the SPACE includes padding to a size_t boundary, whereas the LEN
does not. We have to allocate the SPACE. Previously, we told the kernel
that the buffer size we wanted was the SPACE, not the LEN, which meant
it was free to fill the padding with additional fds: on a 64-bit
platform with 32-bit int, that's one extra fd, if *n_fds happens
to be odd.

This meant that a malicious sender could send exactly 1 fd too many,
which would make us fail an assertion if enabled, or overrun a buffer
by 1 fd otherwise.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83622
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>