Various fixes for away message handling

This makes away message handling more stable and reliable.

Some detailed changes:
*  doc/dbus-interface.html: update D-Bus documentation with the away message parameter
*  src/cinnamon-screensaver-command.c: just send an empty text if no custom away message specified; fix message type (gchar instead of gboolean)
* src/cinnamon-screensaver-dialog.c: remove unused variable
* src/gs-listener-dbus.c: when the lock is called by logind/consolekit, there is no custom away message, so we should send a LOCK signal directly with an empty away message
*  src/gs-manager.c: fix a double free crash, which caused by a copy-paste mistake (logout_command freed instead of away_message); initialize away message properly; don't fail if away_message is NULL
* src/gs-window-x11.c: remove unused function (str_replace); escape away message with g_markup_escape_text, so users can safely use &, < and > characters; don't fail if away_message is NULL; initialize away message properly