Execute /etc/X11/Xsession if available

Ubuntu already provides a /etc/X11/Xsession script which executes
the /etc/X11/Xsession.d scripts.

Those scripts already do the Xresource merge and other actions.

The last script from /etc/X11/Xsession.d executes the session
command specified by $STARTUP, if that is empty it will try
to determine it by itself with the risk of executing something
else.

With this patch we let distros shipping /etc/X11/Xsession to
execute the designated session command and handle Xresource
merge and stuff like that by itself.

At least two other major distributions such as Fedora and
ArchLinux do not have /etc/X11/Xsession, in that case we
will keep the Xresource merge code path and run the session
command ourselves.

Fixes #449

data/scripts/Xsession

@@ -57,25 +57,14 @@ if [ -d /etc/X11/xinit/xinitrc.d ]; then
   done
 fi
 
-# Load Xsession scripts
-# OPTIONFILE, USERXSESSION, USERXSESSIONRC and ALTUSERXSESSION are required
-# by the scripts to work
-xsessionddir="/etc/X11/Xsession.d"
-OPTIONFILE=/etc/X11/Xsession.options
-USERXSESSION=$HOME/.xsession
-USERXSESSIONRC=$HOME/.xsessionrc
-ALTUSERXSESSION=$HOME/.Xsession
-
-if [ -d "$xsessionddir" ]; then
-    for i in `ls $xsessionddir`; do
-        script="$xsessionddir/$i"
-        echo "Loading X session script $script"
-        if [ -r "$script"  -a -f "$script" ] && expr "$i" : '^[[:alnum:]_-]\+$' > /dev/null; then
-            . "$script"
-        fi
-    done
+# Load Xsession scripts, if any
+if [ -x /etc/X11/Xsession ]; then
+    # Let Xsession execute the session for us
+    export STARTUP=$session
+    exec /etc/X11/Xsession
 fi
 
+# Merge Xresources
 if [ -d /etc/X11/Xresources ]; then
   for i in /etc/X11/Xresources/*; do
     [ -f $i ] && xrdb -merge $i
@@ -85,6 +74,7 @@ elif [ -f /etc/X11/Xresources ]; then
 fi
 [ -f $HOME/.Xresources ] && xrdb -merge $HOME/.Xresources
 
+# Execute the actual session
 case $session in
   "")
     exec xmessage -center -buttons OK:0 -default OK "Sorry, $DESKTOP_SESSION is no valid session."