Teach extra::term::Terminal.reset() to handle missing op

Unlike fg() and bg(), we haven't already checked for the presence of
"op" in the terminfo when we call reset(), so we need to handle the case
where it's missing.

Also update the warn!() lines to avoid double-quoting the output.

Fixes #7431.
auto merge of #7436 : kballard/rust/term-dumb, r=cmr

Unlike fg() and bg(), we haven't already checked for the presence of
"op" in the terminfo when we call reset(), so we need to handle the case
where it's missing.

Also update the warn!() lines to avoid double-quoting the output.

Fixes #7431.