hush: add support for "set -o pipefail"

function                                             old     new   delta
checkjobs                                            467     517     +50
builtin_set                                          259     286     +27
o_opt_strings                                          -      10     +10
hush_main                                           1011    1013      +2
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/0 up/down: 89/0)               Total: 89 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/pipefail.right b/shell/hush_test/hush-misc/pipefail.right
new file mode 100644
index 0000000..5845d89
--- /dev/null
+++ b/shell/hush_test/hush-misc/pipefail.right
@@ -0,0 +1,40 @@
+Default:
+true | true:
+0
+1
+true | false:
+1
+0
+false | true:
+0
+1
+exit 2 | exit 3 | exit 4:
+4
+0
+Pipefail on:
+true | true:
+0
+1
+true | false:
+1
+0
+false | true:
+1
+0
+exit 2 | exit 3 | exit 4:
+4
+0
+Pipefail off:
+true | true:
+0
+1
+true | false:
+1
+0
+false | true:
+0
+1
+exit 2 | exit 3 | exit 4:
+4
+0
+Done