Mail: support SASL EXTERNAL (RFC 4422). This is needed to allow TLS client certificate auth to work. With ssl_verify_client configured, the auth daemon can choose to allow the connection to proceed based on the certificate data. This has been tested with Thunderbird for IMAP only. I've not yet found a client that will do client certificate auth for POP3 or SMTP, and the method is not really documented anywhere that I can find. That said, its simple enough that the way I've done is probably right.

--- a/src/mail/ngx_mail_auth_http_module.c
+++ b/src/mail/ngx_mail_auth_http_module.c
@@ -151,6 +151,7 @@ static ngx_str_t   ngx_mail_auth_http_me
     ngx_string("plain"),
     ngx_string("apop"),
     ngx_string("cram-md5"),
+    ngx_string("external"),
     ngx_string("none")
 };