pop3: Fixed auth preference not being honored when CAPA not supported

If a user indicated they preferred to authenticate using APOP or a SASL
mechanism, but neither were supported by the server, curl would always
fall back to clear text when CAPA wasn't supported, even though the
user didn't want to use this.

This also fixes the auto build failure caused by commit 6f2d5f0562f64a.
imap: Fixed auth preference not being honored when CAPABILITY not supported

If a user indicated they preferred to authenticate using a SASL
mechanism, but SASL authentication wasn't supported by the server, curl
would always fall back to clear text when CAPABILITY wasn't supported,
even though the user didn't want to use this.