Fix #80710: imap_mail_compose() header injection

Like `mail()` and `mb_send_mail()`, `imap_mail_compose()` must prevent
header injection. For maximum backward compatibility, we still al

Fix #80710: imap_mail_compose() header injection

Like `mail()` and `mb_send_mail()`, `imap_mail_compose()` must prevent
header injection. For maximum backward compatibility, we still allow
header folding for general headers, and still accept trailing line
breaks for address lists.

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