Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibC/errno.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <errno_numbers.h>
+#include <sys/cdefs.h>
+
+#define __RETURN_WITH_ERRNO(rc, good_ret, bad_ret) \
+    do {                                           \
+        if (rc < 0) {                              \
+            errno = -rc;                           \
+            return (bad_ret);                      \
+        } else {                                   \
+            errno = 0;                             \
+            return (good_ret);                     \
+        }                                          \
+    } while (0)
+
+__BEGIN_DECLS
+
+extern const char* sys_errlist[];
+extern int sys_nerr;
+extern int errno;
+
+__END_DECLS