(empth_create): LWP yields, and it's the reference implementation.

Document and fix the others.

include/empthread.h

@@ -131,6 +131,7 @@ int empth_init(void **ctx, int flags);
  * UD is the value to pass to ENTRY.  It is also assigned to the
  * context variable defined with empth_init() whenever the thread gets
  * scheduled.
+ * Yield the processor.
  * Return the thread, or NULL on error.
  */
 empth_t *empth_create(int prio, void (*entry)(void *),

src/lib/empthread/ntthread.c

@@ -463,6 +463,7 @@ empth_create(int prio, void (*entry)(void *), int size, int flags,
     }
 
     loc_debug("new thread id is %ld", pThread->ulThreadID);
+    empth_yield();
     return pThread;
 
   bad:

src/lib/empthread/pthread.c

@@ -236,6 +236,7 @@ empth_create(int prio, void (*entry)(void *), int size, int flags,
 	goto bad;
     }
     empth_status("new thread id is %ld", (long)t);
+    empth_yield();
     return ctx;
 
   bad: