lei: drop EOFpipe in favor of PktOp

lei: drop EOFpipe in favor of PktOp

lei already uses PktOp and SOCK_SEQPACKET throughout; whereas
EOFpipe had one single use in lei.  Since PktOp is a strict
superset of EOFpipe functionality, we may be able to get rid of
EOFpipe entirely.

However, lei is considered a portability canary and I'm not sure
if the stable public-inbox-* code can drop EOFpipe just yet.