* fixes#15770
* When running under the UTF-8 codepage, Powershell subprocesses will fail (eg, Start-Job, others) if the input encoding is using the default BOM preamble. This fix forces it to use no preamble in leaf_exec and win_shell, and includes tests to verify that Start-Job works.
* fixes#22441
* fixes#22655
* moves all env handling into the exec wrapper; this should work for everything but raw, which is consistent with non-Windows.
