Escape scripting error messages so they are valid markup

Move generic keybindings (shift/alt/ctrl/meta/super) into new
D_Responder handler

Only take a screenshot if the menu and console have passed on the event;
this actually fixes the screenshot bug mentioned in the source

Reformat M_Responder

Convert HU_Responder and M_Responder to use the keybindings global
instead of static shiftdown (etc.) variables