NOFORK fixes

"rm -i FILE" and "yes" can now be interrupted by ^C in hush.
This also now works:

$ usleep 19999999
^C
$ echo $?
130

function                                             old     new   delta
run_pipe                                            1668    1711     +43
pseudo_exec_argv                                     312     321      +9
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 52/0)               Total: 52 bytes

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

coreutils/usleep.c

@@ -38,6 +38,13 @@ int usleep_main(int argc UNUSED_PARAM, char **argv)
 		bb_show_usage();
 	}
 
+	/* Safe wrt NOFORK? (noforks are not allowed to run for
+	 * a long time). Try "usleep 99999999" + ^C + "echo $?"
+	 * in hush with FEATURE_SH_NOFORK=y.
+	 * At least on uclibc, usleep() thanslates to nanosleep()
+	 * which returns early on any signal (even caught one),
+	 * and uclibc does not loop back on EINTR.
+	 */
 	usleep(xatou(argv[1]));
 
 	return EXIT_SUCCESS;