shell: add comments about SIGINT-related problems

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/shell_common.c

@@ -196,6 +196,7 @@ shell_builtin_read(struct builtin_read_params *params)
 		 */
 		errno = 0;
 		pfd[0].events = POLLIN;
+//TODO race with a signal arriving just before the poll!
 		if (poll(pfd, 1, timeout) <= 0) {
 			/* timed out, or EINTR */
 			err = errno;