Add force_xinerama configuration option

The configuration option does the same as the commandline parameter, except
it can be easily set by the user (e.g. you are using KDM and can't start a
session through ~/.xsession).

Signed-off-by: Michael Walle <michael@walle.cc>

src/cfgparse.l

@@ -125,6 +125,7 @@ none                            { return TOK_NONE; }
 1pixel                          { return TOK_1PIXEL; }
 focus_follows_mouse             { return TOKFOCUSFOLLOWSMOUSE; }
 force_focus_wrapping            { return TOK_FORCE_FOCUS_WRAPPING; }
+force_xinerama                  { return TOK_FORCE_XINERAMA; }
 workspace_bar                   { return TOKWORKSPACEBAR; }
 popup_during_fullscreen         { return TOK_POPUP_DURING_FULLSCREEN; }
 ignore                          { return TOK_IGNORE; }