implement pthread_condattr_* interfaces

note that, while the attributes are stored, they are not used in
pthread_cond_init yet.

src/thread/pthread_condattr_init.c

@@ -0,0 +1,7 @@
+#include "pthread_impl.h"
+
+int pthread_condattr_init(pthread_condattr_t *a)
+{
+	memset(a, 0, sizeof *a);
+	return 0;
+}