(PUP-1404) execpipe resets LANG and LC_ALL

Puppet::Util::Execution.execpipe always run commands with LANG="C" and
LC_ALL="C" to avoid localized output.
(PUP-1404) Add tests for execpipe locale setting
(PUP-1404) Merge execpipe locale settings (#2234).

This merges in the changes to execpipe to reset LANG and LC_ALL to C in
order to get default program output, and tests for the changes.