Reject control characters in IMAP commands

[sethmlarson]

Linked PRs

• gh-143921: Reject control characters in IMAP commands #143922
• gh-143921: Narrow the control character check in imaplib commands #153067
• [3.15] gh-143921: Narrow the control character check in imaplib commands (GH-153067) #153135
• [3.14] gh-143921: Reject NUL, CR and LF in IMAP commands (GH-143922, GH-153067) #153137
• [3.13] gh-143921: Reject NUL, CR and LF in IMAP commands (GH-143922, GH-153067) (GH-153137) #153287

[bitdancer]

If there is someone familiar enough with IMAP to review this, please do.

Otherwise I need a bit more detail, since I'm not that familiar with IMAP, and both the RFC and the code are quite complex. Can you cover how the RFC makes non-printables (other than space) illegal in 'commands', and how that relates to the imaplib implementation of _command? I scanned the RFC and code for a bit, and while my gut says your PR is correct, it isn't obvious by any means.