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;
+}

src/thread/pthread_setconcurrency.c

@@ -0,0 +1,9 @@
+#include <pthread.h>
+#include <errno.h>
+
+int pthread_setconcurrency(int val)
+{
+	if (val < 0) return EINVAL;
+	if (val > 0) return EAGAIN;
+	return 0;
+}