lei: require Socket::MsgHdr or Inline::C, drop oneshot
The cost of supporting separate code paths between oneshot and
daemon isn't worth the trouble; especially if there are more
users to support. The test suite time nearly doubles with
oneshot, so that's hurting developer productivity.
FD passing is currently required to work efficiently with
remote HTTP(S) queries which return large messages, as seen in
commit
708b182a57373172f5523f3dc297659d58e03b58
("ipc: wq: handle >MAX_ARG_STRLEN && <EMSGSIZE case").
Additionally, upcoming support for IMAP IDLE and inotify-based
monitoring of Maildirs cannot work properly without a background
daemon.