New file to deal with STLS.

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000, 2001, 2003 Free Software Foundation, Inc.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include "../md5.h"
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+
+#include <mailutils/sys/pop3.h>
+#include <mailutils/tls.h>
+
+/*
+ * STLS
+ * We have to assume that the caller check the CAPA and TLS was supported.
+ */
+int
+mu_pop3_stls (mu_pop3_t pop3)
+{
+#ifdef WITH_TLS
+  int status;
+
+  /* Sanity checks.  */
+  if (pop3 == NULL)
+    {
+      return EINVAL;
+    }
+
+  switch (pop3->state)
+    {
+    case MU_POP3_NO_STATE:
+      status = mu_pop3_writeline (pop3, "STLS\r\n");
+      MU_POP3_CHECK_ERROR (pop3, status);
+      mu_pop3_debug_cmd (pop3);
+      pop3->state = MU_POP3_STLS;
+
+    case MU_POP3_STLS:
+      status = mu_pop3_send (pop3);
+      MU_POP3_CHECK_EAGAIN (pop3, status);
+      pop3->acknowledge = 0;
+      pop3->state = MU_POP3_STLS_ACK;
+
+    case MU_POP3_STLS_ACK:
+      {
+        stream_t tls_stream;
+        status = mu_pop3_response (pop3, NULL, 0, NULL);
+        MU_POP3_CHECK_EAGAIN (pop3, status);
+        mu_pop3_debug_ack (pop3);
+        MU_POP3_CHECK_OK (pop3);
+        status = tls_stream_create_client_from_tcp (&tls_stream, pop3->carrier, 0);
+        MU_POP3_CHECK_ERROR (pop3, status);
+        pop3->carrier = tls_stream; 
+        pop3->state = MU_POP3_STLS_CONNECT;
+        break;
+      }
+
+    case MU_POP3_STLS_CONNECT:
+      status = stream_open (pop3->carrier);
+      MU_POP3_CHECK_EAGAIN (pop3, status);
+      pop3->state = MU_POP3_NO_STATE;
+      break;
+
+      /* They must deal with the error first by reopening.  */
+    case MU_POP3_ERROR:
+      status = ECANCELED;
+      break;
+
+      /* No case in the switch another operation was in progress.  */
+    default:
+      status = EINPROGRESS;
+    }
+
+  return status;
+#else
+  (void)pop3;
+  return ENOTSUP;
+#endif
+}