ssh: sanitize the banner displayed by BannerDisplayStderr

[CaptainTed1]

A malicious server can embed terminal control characters and escape
sequences in the pre-authentication banner, which BannerDisplayStderr
writes verbatim to stderr. Strip control characters, other than tab,
carriage return, and newline, before displaying the banner, matching
OpenSSH's behavior. Users who need the unfiltered banner can provide
their own BannerCallback.

Fixes golang/go#80302

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