ash: fix ash-signals/signal8 testcase failure

function                                             old     new   delta
killcmd                                              109     224    +115
kill_main                                            882     910     +28
changepath                                           194     195      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 144/0)             Total: 144 bytes

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

shell/ash_test/ash-signals/sigint1.right

@@ -0,0 +1 @@
+Sending SIGINT to main shell PID

shell/ash_test/ash-signals/sigint1.tests

@@ -0,0 +1,41 @@
+# What should happen if non-interactive shell gets SIGINT?
+
+(sleep 1; echo Sending SIGINT to main shell PID; exec kill -INT $$) &
+
+# We create a child which exits with 0 even on SIGINT
+# (This is truly necessary only if SIGINT is generated by ^C,
+# in this testcase even bare "sleep 2" would do because
+# we don't send SIGINT _to_ the_ child_...)
+$THIS_SH -c 'trap "exit 0" SIGINT; sleep 2'
+
+# In one second, we (main shell) get SIGINT here.
+# The question is whether we should, or should not, exit.
+
+# bash will not stop here. It will execute next command(s).
+
+# The rationale for this is described here:
+# http://www.cons.org/cracauer/sigint.html
+#
+# Basically, bash will not exit on SIGINT immediately if it waits
+# for a child. It will wait for the child to exit.
+# If child exits NOT by dying on SIGINT, then bash will not exit.
+#
+# The idea is that the following script:
+# | emacs file.txt
+# | more cmds
+# User may use ^C to interrupt editor's ops like search. But then
+# emacs exits normally. User expects that script doesn't stop.
+#
+# This is a nice idea, but detecting "did process really exit
+# with SIGINT?" is racy. Consider:
+# | bash -c 'while true; do /bin/true; done'
+# When ^C is pressed while bash waits for /bin/true to exit,
+# it may happen that /bin/true exits with exitcode 0 before
+# ^C is delivered to it as SIGINT. bash will see SIGINT, then
+# it will see that child exited with 0, and bash will NOT EXIT.
+
+# Therefore we do not implement bash behavior.
+# I'd say that emacs need to put itself into a separate pgrp
+# to isolate shell from getting stray SIGINTs from ^C.
+
+echo Next command after SIGINT was executed