Add dwm as a submodule.
X: scripts: add startwm script.

This is a workaround for being able to start the wm and automatically
loaded programs within a shell that knows the ssh-agent's PID.  From
xinitrc, this script is run with ssh-agent, and the PID is past down to
the children of the script instance.  This allows me to start programs
before exec'ing the window manager executable with connection to ssh-agent.