sort: use noted(NDFLT) in note handler

There are many more random notes flying around
here in Unix-land than there were on Plan 9.
For example, some shells implement "cat file | sort"
with cat as the child of sort, so that when cat exits,
sort gets a "sys: child" note.  noted(NDFLT) knows
which signals aren't really important and can be ignored,
and which need to kill the program.

src/cmd/sort.c

@@ -243,8 +243,7 @@ notifyf(void *a, char *s)
 		done(0);
 	if(strncmp(s, "sys: write on closed pipe", 25) == 0)
 		done(0);
-	fprint(2, "sort: note: %s\n", s);
-	abort();
+	noted(NDFLT);
 }
 
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