add pipe2 syscall

based on patch by orc and Isaac Dunham, with some details fixed.

include/unistd.h

@@ -174,6 +174,7 @@ int setresgid(gid_t, gid_t, gid_t);
 int getresuid(uid_t *, uid_t *, uid_t *);
 int getresgid(gid_t *, gid_t *, gid_t *);
 char *get_current_dir_name(void);
+int pipe2(int [2], int);
 #endif
 
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)

src/linux/pipe2.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int pipe2(int fd[2], int flg)
+{
+	return syscall(SYS_pipe2, fd, flg);
+}