streamline old-kernel fallback path of pipe2 to use syscalls directly

also, don't waste code/time on F_GETFL since pipes always have blank
flags initially (at least on old kernels, which are all this fallback
code matters for).

src/unistd/pipe2.c

@@ -11,12 +11,12 @@ int pipe2(int fd[2], int flag)
 	ret = __syscall(SYS_pipe, fd);
 	if (ret) return __syscall_ret(ret);
 	if (flag & O_CLOEXEC) {
-		fcntl(fd[0], F_SETFD, FD_CLOEXEC);
-		fcntl(fd[1], F_SETFD, FD_CLOEXEC);
+		__syscall(SYS_fcntl, fd[0], F_SETFD, FD_CLOEXEC);
+		__syscall(SYS_fcntl, fd[1], F_SETFD, FD_CLOEXEC);
 	}
 	if (flag & O_NONBLOCK) {
-		fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL) | O_NONBLOCK);
-		fcntl(fd[1], F_SETFL, fcntl(fd[1], F_GETFL) | O_NONBLOCK);
+		__syscall(SYS_fcntl, fd[0], F_SETFL, O_NONBLOCK);
+		__syscall(SYS_fcntl, fd[1], F_SETFL, O_NONBLOCK);
 	}
 	return 0;
 }