IO::Uncompress::Gunzip seems to be losing $? when closing
PublicInbox::ProcessPipe. To workaround this, do a synchronous
waitpid ourselves to force proper $? reporting update tests to
use the new --only feature for testing invalid URLs.
This improves internal code consistency by having {pkt_op}
parse the same ASCII-only protocol script/lei understands.
We no longer pass {sock} to worker processes at all,
further reducing FD pressure on per-user limits.