[kernel] Add thread_join syscall

Thread joining is an important primitive that I seem to have totally
forgotten to implement previously.

definitions/objects/thread.def

@@ -3,6 +3,7 @@ object thread : object {
 
     capabilities [
         kill
+        join
     ]
 
     method create [constructor] {
@@ -13,6 +14,8 @@ object thread : object {
 
     method kill [destructor cap:kill]
 
+    method join [cap:join]
+
     method exit [static]
 
     method sleep [static] {