two chroot related fixes

- Pass SCR root path to stdio agents [bnc#879365](https://bugzilla.novell.com/show_bug.cgi?id=879365).
- If aborting on a SCRAgent::targetPath assertion, log it [bnc#891053](https://bugzilla.novell.com/show_bug.cgi?id=891053)

I did clean all places using it. Note that it is a private header, not present in yast2-core-devel.rpm. So I consider it a safe change[TM].