email: Added mutual authentication flag

lib/imap.h

@@ -92,6 +92,7 @@ struct imap_conn {
   bool tls_supported;         /* StartTLS capability supported by server */
   bool login_disabled;        /* LOGIN command disabled by server */
   bool ir_supported;          /* Initial response supported by server */
+  bool mutual_auth;           /* Mutual authentication enabled (GSSAPI only) */
   char *mailbox;              /* The last selected mailbox */
   char *mailbox_uidvalidity;  /* UIDVALIDITY parsed from select response */
 };

lib/pop3.h

@@ -84,6 +84,7 @@ struct pop3_conn {
   unsigned int authused;  /* SASL auth mechanism used for the connection */
   char *apoptimestamp;    /* APOP timestamp from the server greeting */
   bool tls_supported;     /* StartTLS capability supported by server */
+  bool mutual_auth;       /* Mutual authentication enabled (GSSAPI only) */
 };
 
 extern const struct Curl_handler Curl_handler_pop3;

lib/smtp.h

@@ -86,6 +86,7 @@ struct smtp_conn {
   bool size_supported;     /* If server supports SIZE extension according to
                               RFC 1870 */
   bool auth_supported;     /* AUTH capability supported by server */
+  bool mutual_auth;        /* Mutual authentication enabled (GSSAPI only) */
 };
 
 extern const struct Curl_handler Curl_handler_smtp;