readline: handle input starting with control chars

[eschrock]

Handle control characters only when there is a single byte in the
stream, otherwise fall through to the standard multibyte handling.

Fixes #5508

[Nodejs-Jenkins]

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The following commiters were not found in the CLA:

• Eric Schrock

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[eschrock]

FYI, I filled out the CLA this morning. Not sure if it hasn't propagated, or fails because I used '@gmail.com' in the CLA but my push uses '@delphix.com'. I will submit a CLA for the latter just in case.

[tjfontaine]

Ya, the bot generally keys on the commit email, before the push though a human usually verifies so it wouldn't have been a huge issue

[bnoordhuis]

LGTM.

/cc @TooTallNate

[tjfontaine]

Thanks! Merged in 35ae696