Spawn was designed to speed up process spawning inside
long-lived daemons with largish memory usage. It does not help
for short-lived scripts which only exist to start and connect to
a daemon.
This change actually speeds up initial lei startup from
~190ms to ~140ms(!). Normal usage once the daemon is running
is unaffected, at <20ms for help text.
While we're in the area, simplify Cwd error message generation,
too.