Fixing color and refactor xsetcolorname.

By  the recommendation of FRIGN I refactored xsetcolorname to remove the
unnecessary r, g, b variables when allocating a new  color.  Colors  are
now freed and set to the new color. A die() should not happen here. Oth‐
erwise it is easy for applications to kill st. St should be resilent  to
malicious input.

Second  this  patch  standardises  the  naming  of  »color«. There is no
»colour« here. Maybe in some parts of the world.