implement pthread_[sg]etconcurrency.

there is a resource limit of 0 bits to store the concurrency level
requested. thus any positive level exceeds a resource limit, resulting
in EAGAIN. :-)
diff --git a/include/pthread.h b/include/pthread.h
index 43754dd..5d97ebf 100644
--- a/include/pthread.h
+++ b/include/pthread.h
@@ -178,6 +178,9 @@
 
 int pthread_atfork(void (*)(void), void (*)(void), void (*)(void));
 
+int pthread_getconcurrency(void);
+int pthread_setconcurrency(int);
+
 #include <bits/pthread.h>
 
 int __setjmp(void *);