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. :-)

include/pthread.h

@@ -178,6 +178,9 @@ int pthread_barrierattr_setpshared(pthread_barrierattr_t *, int);
 
 int pthread_atfork(void (*)(void), void (*)(void), void (*)(void));
 
+int pthread_getconcurrency(void);
+int pthread_setconcurrency(int);
+
 #include <bits/pthread.h>
 
 int __setjmp(void *);