smtp: Added support for NTLM authentication

Modified smtp_endofresp() to detect NTLM from the server specified list
of supported authentication mechanisms.

Modified smtp_authenticate() to start the sending of the NTLM data.

Added smtp_auth_ntlm_type1_message() which creates a NTLM type-1
message. This function is used by authenticate() to start the sending
of data and by smtp_state_auth_ntlm_resp() when the AUTH command
doesn't contain the type-1 message as part of the initial response.
This lack of initial response can happen if an OOM error occurs or the
type-1 message is longer than 504 characters. As the main AUTH command
is limited to 512 character the data has to be transmitted in two
parts; one containing the AUTH NTLM and the second containing the
type-1 message.

Added smtp_state_auth_ntlm_type2msg_resp() which handles the incoming
type-2 message and sends an outgoing type-3 message. This type-2
message is sent by the server in response to our type-1 message.

Modified smtp_state_auth_resp() to handle the response to: the AUTH
NTLM without the initial response and the type-2 response.

Modified smtp_disconnect() to cleanup the NTLM SSPI stack.

lib/smtp.h

@@ -40,6 +40,8 @@ typedef enum {
   SMTP_AUTHLOGIN,
   SMTP_AUTHPASSWD,
   SMTP_AUTHCRAM,
+  SMTP_AUTHNTLM,
+  SMTP_AUTHNTLM_TYPE2MSG,
   SMTP_AUTH,
   SMTP_MAIL, /* MAIL FROM */
   SMTP_RCPT, /* RCPT TO */
@@ -57,6 +59,7 @@ struct smtp_conn {
   size_t eob;         /* number of bytes of the EOB (End Of Body) that has been
                          received thus far */
   unsigned int authmechs;       /* Accepted authentication methods. */
+  unsigned int authused;  /* Authentication method used for the connection */
   smtpstate state; /* always use smtp.c:state() to change state! */
   struct curl_slist *rcpt;
   bool ssldone; /* is connect() over SSL done? only relevant in multi mode */
@@ -69,6 +72,7 @@ struct smtp_conn {
 #define SMTP_AUTH_DIGEST_MD5    0x0008
 #define SMTP_AUTH_GSSAPI        0x0010
 #define SMTP_AUTH_EXTERNAL      0x0020
+#define SMTP_AUTH_NTLM          0x0040
 
 extern const struct Curl_handler Curl_handler_smtp;
 extern const struct Curl_handler Curl_handler_smtps;