tests/unix_seqpacket: provide random data pumping test with MSG_EOR

Allocate a big chunk of randomly initialized memory. Send it to the peer
in random sized chunks, throwing MSG_EOR at randomly initialized offsets.
Receive into random sized chunks setting MSG_WAITALL randomly. Check that
MSG_EORs where they should be, check that MSG_WAITALL is abode, but
overriden by MSG_EOR. And finally memcmp() what we receive.

Reviewed by: asomers, tuexen
Differential Revision: https://reviews.freebsd.org/D43775