papi: Use CMSG_SPACE for sizing ancillary buffer space

CMSG_LEN provides the length required to store an ancillary item, but it
does not consider padding. RFC 3542 requires portable applications to
use CMSG_SPACE(), this is documented in the python API:

https://docs.python.org/3/library/socket.html#socket.CMSG_LEN

Type: improvement
Change-Id: I0eab470585f2f39bea38639db95d6b0e047d2cff
Signed-off-by: Tom Jones <thj@freebsd.org>