bpo-34521: Fix tests in test_multiprocessing_spawn to use correctly CMSG_SPACE (GH-9613)

After some failures in AMD64 FreeBSD CURRENT Debug 3.x buildbots
regarding tests in test_multiprocessing_spawn and after examining
similar failures in test_socket, some errors in the calculation of
ancillary data buffers were found in multiprocessing.reduction.

CMSG_LEN() can often be used as the buffer size for recvmsg() to
receive a single item of ancillary data, but RFC 3542 requires portable
applications to use CMSG_SPACE() and thus include space for padding,
even when the item will be the last in the buffer.

The failures we experience are due to the usage of CMSG_LEN() instead of
CMSG_SPACE().