build-sys: use LN_S in Makefile.am

For consistency only. We're Linux only, so it really makes no
difference.

https://bugs.freedesktop.org/show_bug.cgi?id=63036
shell-completion: work on session shell is --user is used

https://bugs.freedesktop.org/show_bug.cgi?id=61695
PAM, uaccess: check for logind, not for systemd

It is possible to build systemd without logind or run logind without systemd
init. Commit 66e41181 fixed sd_booted() to only succeed for systemd init; with
that, testing for systemd init is wrong in the parts that talk to logind.

In particular, this affects the PAM module and the "uaccess" udev builtin.
Change sd_booted() to a new logind_running() which tests for
/run/systemd/seats/.

For details, see:
<https://mail.gnome.org/archives/desktop-devel-list/2013-March/msg00092.html>

https://bugs.freedesktop.org/show_bug.cgi?id=62754