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/network/accept4.c

@@ -0,0 +1,9 @@
+#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);
+}