Normative: formalize "interaction.pressKeys" steps

[jugglinmike]

• Reference the input parameters by name rather than by type.
• Delineate implementation-defined behavior in dedicated steps.
• Specify a return value for successful operations.

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[jugglinmike]

Since submitting this patch, I've recognized that the algorithm is also ambiguous about the error it reports when the system cannot simulate keyboard interaction. Although the factors that define this state are necessarily implementation-defined, the error reported to the local end ought to be standardized. I've pushed up an addition commit that defines a new error type, references that type from a new "Security Check" algorithm (substantiating the existing section reserved for such prose), and invokes that algorithm from "interaction.pressKeys".

[jugglinmike]

Thanks, @jkva!