x11/lightdm: Make Xsession check for more system-wide config files

Xresources, Xkbmap and Xmodmap will now be read if they are found in
%%LOCALBASE%%/etc/X11/ or %%LOCALBASE%%/etc/X11/xinit/
with or without a "." prefix.

User-specific versions are still also found in $HOME/ with a "." prefix.