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. :-)

src/thread/pthread_getconcurrency.c

@@ -0,0 +1,6 @@
+#include <pthread.h>
+
+int pthread_getconcurrency()
+{
+	return 0;
+}