Bugfix: Use both parts of WM_CLASS (it contains instance and class) (Thanks fallen)

Bugfix: Use both parts of WM_CLASS (it contains instance and class) (Thanks fallen)

Actually, WM_CLASS contains two null-terminated strings, so we cannot
use asprintf() to get its value but rather use strdup() to get both
of them. Both values are compared when a client is matched against
a wm_class/title combination (for assignments for example).