bugfix for shift modifier setting in xwin-grab-keys.

The code responsible for adding the shift modifier to the keys that need it in the xwin-grab-keys function setfs key, causing it to change for all codes looped over.
This was preventing me from being able to set the super key as the prefix key, as Super_L has two associated codes, the first of which (206) causes the shift modifier to be added to key, which is then still present for the second code (133) which should not have the shift modifier added.

This change alters the code so that key is rebound with a let rather than changed with setf, preventing the above scenario.