Don't setsid Scheme.

Don't setsid Scheme.

If you want to detach from the controlling terminal, use a trivial
wrapper program that calls setsid and then exec (e.g., pgrphack from
daemontools), or add a primitive to Scheme to daemonize without race
conditions.

Hitting ^C at the terminal now works to interrupt Scheme in batch
mode, e.g. when run under make during the build.