ash: exit after subshell error when errexit option is set

When "set -e" option is on, shell must exit when any command fails,
including compound commands of the form (compound-list) executed in a
subshell. Bash and dash shells have this behaviour.

Also add a corresponding testcase.

Signed-off-by: Rostislav Skudnov <rostislav@tuxera.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash_test/ash-misc/errexit1.tests

@@ -0,0 +1,5 @@
+set -e
+(true)
+echo OK
+(false)
+echo FAIL