move accept4, dup3, and pipe2 to non-linux-specific locations

these interfaces have been adopted by the Austin Group for inclusion
in the next version of POSIX.

src/linux/accept4.c

@@ -1,9 +0,0 @@
-#define _GNU_SOURCE
-#include <sys/socket.h>
-#include "syscall.h"
-#include "libc.h"
-
-int accept4(int fd, struct sockaddr *restrict addr, socklen_t *restrict len, int flg)
-{
-	return socketcall_cp(accept4, fd, addr, len, flg, 0, 0);
-}

src/linux/dup3.c

@@ -1,10 +0,0 @@
-#define _GNU_SOURCE
-#include <unistd.h>
-#include <errno.h>
-#include "syscall.h"
-
-int dup3(int old, int new, int flags) {
-	int r;
-	while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
-	return __syscall_ret(r);
-}

src/linux/pipe2.c

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