BuildSystem: Move all userpace libraries under the userspace directory

As the number of libraries is increasing, root directory starts to
expand. This adds better organization for libraries

userspace/libraries/LibC/include/mqueue.h

@@ -0,0 +1,41 @@
+#ifndef _MQUEUE_H
+#define _MQUEUE_H 1
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/mqueue.h.html
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#include <time.h>
+#include <signal.h>
+
+#define __need_pthread_attr_t
+#define __need_size_t
+#define __need_ssize_t
+#include <sys/types.h>
+
+typedef int mqd_t;
+
+struct mq_attr
+{
+	long mq_flags;    /* Message queue flags. */
+	long mq_maxmsg;   /* Maximum number of messages. */
+	long mq_msgsize;  /* Maximum message size. */
+	long mq_curmsgs;  /* Number of messages currently queued. */
+};
+
+int		mq_close(mqd_t mqdes);
+int		mq_getattr(mqd_t mqdes, struct mq_attr* mqstat);
+int		mq_notify(mqd_t mqdes, const struct sigevent* notification);
+mqd_t	mq_open(const char* name, int oflag, ...);
+ssize_t	mq_receive(mqd_t mqdes, char* msq_ptr, size_t msq_len, unsigned* msg_prio);
+int		mq_send(mqd_t mqdes, const char* msg_ptr, size_t msg_len, unsigned msg_prio);
+int		mq_setattr(mqd_t mqdes, const struct mq_attr* __restrict mqstat, struct mq_attr* __restrict omqstat);
+ssize_t	mq_timedreceive(mqd_t mqdes, char* __restrict msg_ptr, size_t msg_len, unsigned* __restrict msg_prio, const struct timespec* __restrict abstime);
+int		mq_timedsend(mqd_t mqdes, const char* msg_ptr, size_t msg_len, unsigned msg_prio, const struct timespec* abstime);
+int		mq_unlink(const char* name);
+
+__END_DECLS
+
+#endif