service: check error from kill when checking ability to track child.
service: check error from kill when checking ability to track child.

We check whether the process exists using kill. We were treating any
error return as meaning the process ID was invalid, but in fact we may
simply not have permission to signal the process (if we are not being
run as root and the service process is suid).

davmac314/dinit,davmac314/dinit,davmac314/dinit