Kernel+LibPthread: pthread_create handles pthread_attr_t

Add an initial implementation of pthread attributes for:
  * detach state (joinable, detached)
  * schedule params (just priority)
  * guard page size (as skeleton) (requires kernel support maybe?)
  * stack size and user-provided stack location (4 or 8 MB only, must be aligned)

Add some tests too, to the thread test program.

Also, LibC: Move pthread declarations to sys/types.h, where they belong.

Libraries/LibC/limits.h

@@ -33,3 +33,5 @@
 #define MB_LEN_MAX 16
 
 #define ARG_MAX 65536
+
+#define PTHREAD_STACK_MIN 65536