pop3: Enhanced the extended authentication mechanism detection

Enhanced the authentication type / mechanism detection in preparation
for the introduction of APOP support.

lib/pop3.h

@@ -57,14 +57,20 @@ struct pop3_conn {
                              have been received so far */
   size_t strip;           /* Number of bytes from the start to ignore as
                              non-body */
+  unsigned int authtypes; /* Supported authentication types */
   unsigned int authmechs; /* Accepted SASL authentication mechanisms */
-  unsigned int authused;  /* Authentication method used for the connection */
+  unsigned int authused;  /* SASL auth mechanism used for the connection */
   pop3state state;        /* Always use pop3.c:state() to change state! */
 };
 
 extern const struct Curl_handler Curl_handler_pop3;
 extern const struct Curl_handler Curl_handler_pop3s;
 
+/* Authentication type flags */
+#define POP3_TYPE_CLEARTEXT 0x0001
+#define POP3_TYPE_APOP      0x0002
+#define POP3_TYPE_SASL      0x0004
+
 /* This is the 5-bytes End-Of-Body marker for POP3 */
 #define POP3_EOB "\x0d\x0a\x2e\x0d\x0a"
 #define POP3_EOB_LEN 5