procd: guard fork_worker calls

procd: guard fork_worker calls

Usually respawn(), askfirst(), askconsole() and rcrespawn() are run only
one time to start a worker child for the given inittab entry.

In case we want to allow calling these functions several times, we need
to ensure that we do not start multiple workers at the same time for the
same inittab item.

For this, we can re-use the remembered pid of the worker child,
however, we need to reset this pid to allow a new instance in case the
previous child exited.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>