setopt: make CURLOPT_KEYPASSWD work for SSH-only builds

This option is used for both TLS and SSH so it needs to be handled
even in TLS-disabled builds

Mention this in the man page as well.

Follow-up to 52fa8d9
Pointed out by Codex Security
Closes #22121

docs/libcurl/opts/CURLOPT_KEYPASSWD.md

@@ -9,6 +9,8 @@ See-also:
   - CURLOPT_SSLKEY (3)
 Protocol:
   - TLS
+  - SFTP
+  - SCP
 TLS-backend:
   - OpenSSL
   - mbedTLS