stty: fix negated options getting rejected by clap

[willshuttleworth]

fix for #6820. currently, there is no way to negate or disable an stty setting. for example, stty echonl works, but it cannot be undone with stty -echonl since clap tries to parse -echonl as an option. allowing hyphen values for settings fixes this.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)