KProcessRunner: only emit processStarted() signal once
As discussed with dfaure on IRC, processStarted() signal should only be emitted once from setPid(). In this code "!m_pid" is used as "this hasn't been run before" condition.
This fixes a crash in applicationlauncherjob_servicetest, which incidentally passes on the CI and when using kdesrc-build; for the crash to happen systemd needs to be actually running/used by KProcessRunner.
This reverts commit aa4e0645.