MFC r344502: sh: Add set -o pipefail

The pipefail option allows checking the exit status of all commands in a
pipeline more easily, at a limited cost of complexity in sh itself. It works
similarly to the option in bash, ksh93 and mksh.

Like ksh93 and unlike bash and mksh, the state of the option is saved when a
pipeline is started. Therefore, even in the case of commands like
A | B &
a later change of the option does not change the exit status, the same way
(A | B) &
works.

Since SIGPIPE is not handled specially, more work in the script is required
for a proper exit status for pipelines containing commands such as head that
may terminate successfully without reading all input. This can be something
like

(
cmd1
r=$?
if [ "$r" -gt 128 ] && [ "$(kill -l "$r")" = PIPE ]; then
exit 0
else
exit "$r"
fi
) | head

PR: 224270
Relnotes: yes