Small fixes for execute_kwallet

Summary:
Don't return on error conditions, exit instead. It's a child process we
need to finish it
Don't use pam_syslog, there's a comment at the beginning saying it
doesn't work, it seems to kind of work for me but some investigation by
Matthias Gerstner seems to show it's doing funny things with the fds
so don't use it.

Reviewers: fvogt

Reviewed By: fvogt

Subscribers: anthonyfieroni, mgerstner, fvogt, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13013