add NOMMU fixme's; move move_fd from runit_lib to libbb; nuke fd_copy

libbb/xfuncs.c

@@ -133,6 +133,16 @@ int ndelay_off(int fd)
 	return fcntl(fd,F_SETFL,fcntl(fd,F_GETFL,0) & ~O_NONBLOCK);
 }
 
+// "Renumber" opened fd
+void xmove_fd(int from, int to)
+{
+	if (from == to)
+		return;
+	if (dup2(from, to) != to)
+		bb_perror_msg_and_die("cannot duplicate file descriptor");
+	close(from);
+}
+
 // Die with an error message if we can't write the entire buffer.
 void xwrite(int fd, const void *buf, size_t count)
 {