Set %ENV keys using the same byte-string logic as setting %ENV values.

Set %ENV keys using the same byte-string logic as setting %ENV values.

Issue #18636: This extends the work from
613c63b465f01af4e535fdc6c1c17e7470be5aad to %ENV keys. Previously
if you assigned an upgraded string as a key in %ENV, the string’s
internal PV representation was sent to the OS. Now the string is
“soft downgraded” before being given to the OS; if the downgrade
fails--i.e., if the string contains code points above 255--then
a warning is printed, and the string’s utf8 is assigned to the
environment, as happens with %ENV values.

A new internal macro, MgSV, is created to facilitate this work.