in ~/.xsession
:
xmodmap -e 'remove Control = Control_L'
xmodmap -e 'remove Control = Control_R'
xmodmap -e 'clear Lock'
xmodmap -e 'keysym Caps_Lock = Control_R'
xmodmap -e 'keysym Control_L = Caps_Lock'
xmodmap -e 'add Control = Control_L'
xmodmap -e 'add Control = Control_R'
xmodmap -e 'keysym Caps_Lock = Print'
Enjoy.