Knot DNS 1.5.2

Update Knot DNS to new upstream release 1.5.2 + replace
pthread_spinlock_t with pthread_mutex_t to compile on the Mac OS X

# fixed version with upstream patch
Could we perhaps wait for that and add the commit as a patch block instead? Don't like us fixing these things in the formula if upstream is about to pursue a different fix. Leave this open until then, though.