Ignore exit code zero from activated services

Ignore exit code zero from activated services

A variety of system components have migrated from legacy init into DBus
service activation.  Many of these system components "daemonize", which
involves forking.  The DBus activation system treated an exit as an
activation failure, assuming that the child process which grabbed the
DBus name didn't run first.

While we're in here, also differentiate in this code path between the
servicehelper (system) versus direct activation (session) paths.  In
the session activation path our error message mentioned a helper
process which was confusing, since none was involved.

Based on a patch and debugging research from Ray Strode <rstrode@redhat.com>