lei: avoid deadlock on inotify/EVFILT_VNODE wakeups

lei: avoid deadlock on inotify/EVFILT_VNODE wakeups

Enqueuing "note-event" requests from the DS event loop must
not wait on workers being able to drain the queue quickly
enough.  Thus we make the SOCK_SEQPACKET writes nonblocking
and rely on the lei-daemon event loop to enqueue writes.

This is a unique problem for "note-event" since it reuses
workers in between commands, while most lei commands currently
fork off new workers.