Add systemd service file and build hooks for mdm.

This commit adds the basic systemd support needed to ensure mdm will
correctly be started on systems using systemd rather then sysV init
as indicated by Debian packaging guidelines.
Added options to limit (enabled by default) and filter (disabled by default) output in .xsession-errors, so it doesn't get bigger than 200KB and/or doesn't show Gtk/Glib/Gio/Glade/GObject warnings/errors
Merge pull request #122 from monsta/master

Fix some leaks
Merge pull request #119 from wrouesnel/systemd-support

Add systemd service file and build hooks for mdm.
Fixed name in debian/mdm.service
Add pam_kwallet to pam configs. (LP: #1305307) (https://github.com/linuxmint/mdm/issues/117)
1.8.0