add renameat2 linux syscall wrapper

This syscall is available since Linux 3.15 and also implemented in
glibc from version 2.28. It is commonly used in filesystem or security
contexts.

Constants RENAME_NOREPLACE, RENAME_EXCHANGE, RENAME_WHITEOUT are
guarded by _GNU_SOURCE as with glibc.

include/stdio.h

@@ -158,6 +158,13 @@ char *ctermid(char *);
 #define L_ctermid 20
 #endif
 
+#if defined(_GNU_SOURCE)
+#define RENAME_NOREPLACE (1 << 0)
+#define RENAME_EXCHANGE  (1 << 1)
+#define RENAME_WHITEOUT  (1 << 2)
+
+int renameat2(int, const char *, int, const char *, unsigned);
+#endif
 
 #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
  || defined(_BSD_SOURCE)

src/linux/renameat2.c

@@ -0,0 +1,11 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include "syscall.h"
+
+int renameat2(int oldfd, const char *old, int newfd, const char *new, unsigned flags)
+{
+#ifdef SYS_renameat
+	if (!flags) return syscall(SYS_renameat, oldfd, old, newfd, new);
+#endif
+	return syscall(SYS_renameat2, oldfd, old, newfd, new, flags);
+}