Keep compatibility with gnome-screensaver

This change fixes the following problems:

1. Fix crashing due logout_command freed instead of away_message

2. Keep the compatibility with gnome-screensaver's D-Bus interface, by reverting back the 'Lock' method to it's original state (not accept parameters), and introduce a new 'LockMessage' method, which supports the newly introduced away message parameter.

This is a requirement for two reasons:

- GNOME 3.8 will use the 'Lock' D-Bus method to handle the lock key (Ctrl+Alt+L), and it will won't work with cinnamon-srceensaver in the current state:
http://git.gnome.org/browse/gnome-settings-daemon/commit/?id=04050b66ba6241d9c986734c37022ade1aff251d

- Arch Linux (and probably other distros) don't want to add conflicting files with the gnome-screensaver package. Instead both packages should work side by side. Since some applications using the 'gnome-screensaver-command --lock' to activate the screensaver (which uses the 'Lock' D-Bus method), the compatibility needs to be preserved.

I added the new D-Bus method to the documentation also.