sysusers: preserve label of /etc/{passwd, group}

These files are specially labeled on SELinux systems, and we need to
preserve that label.
Add function to open temp files in selinux mode
man: add systemd-coredump(8) and a bunch of links
man: sysusers.d correct default user shell

For the non-root user sysusers uses nologin as the default shell, not
login. Correct the documentation to match the code.
man: mention XDG_DATA_HOME in systemd.unit
path-lookup: don't make ~/.local/share/systemd/user a symlink

We already encourage upstreams to keep the default configuration
separate from user customizations for software that is installed in
the system location. Let's allow that separation also for software
that is installed in the home directory.

Some discussion:
http://thread.gmane.org/gmane.comp.sysutils.systemd.devel/19627