sasl: Implement SASL authorisation identity via CURLOPT_SASL_AUTHZID

Added the ability for the calling program to specify the authorisation
identity (authzid), the identity to act as, in addition to the
authentication identity (authcid) and password when using SASL PLAIN
authentication.

Fixes #3653
Closes #3790

NOTE: This commit was cherry-picked and is part of a series of commits
that added the authzid feature for upcoming 7.66.0. The series was
temporarily reverted in db8ec1f so that it would not ship in a 7.65.x
patch release.

Closes https://github.com/curl/curl/pull/4186

2 files changed, 65 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_SASL_AUTHZID.3 b/docs/libcurl/opts/CURLOPT_SASL_AUTHZID.3
new file mode 100644
index 000000000..65445475d
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_SASL_AUTHZID.3
@@ -0,0 +1,64 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_SASL_AUTHZID 3 "11 Sep 2019" "libcurl 7.66.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_SASL_AUTHZID \- authorisation identity (identity to act as)
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SASL_AUTHZID, char *authzid);
+.SH DESCRIPTION
+Pass a char * as parameter, which should be pointing to the zero terminated
+authorisation identity (authzid) for the transfer. Only applicable to the PLAIN
+SASL authentication mechanism where it is optional.
+
+When not specified only the authentication identity (authcid) as specified by
+the username will be sent to the server, along with the password. The server
+will derive a authzid from the authcid when not provided, which it will then
+uses internally.
+
+When the authzid is specified, the use of which is server dependent, it can be
+used to access another user's inbox, that the user has been granted access to,
+or a shared mailbox for example.
+.SH DEFAULT
+blank
+.SH PROTOCOLS
+IMAP, POP3 and SMTP
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "imap://example.com/");
+  curl_easy_setopt(curl, CURLOPT_USERNAME, "Kurt");
+  curl_easy_setopt(curl, CURLOPT_PASSWORD, "xipj3plmq");
+  curl_easy_setopt(curl, CURLOPT_SASL_AUTHZID, "Ursel");
+  ret = curl_easy_perform(curl);
+  curl_easy_cleanup(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.66.0
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+.SH "SEE ALSO"
+.BR CURLOPT_USERNAME "(3), " CURLOPT_PASSWORD "(3), ".BR CURLOPT_USERPWD "(3)"
diff --git a/docs/libcurl/opts/Makefile.inc b/docs/libcurl/opts/Makefile.inc
index 5460b2a57..094c58f42 100644
--- a/docs/libcurl/opts/Makefile.inc
+++ b/docs/libcurl/opts/Makefile.inc
@@ -273,6 +273,7 @@ man_MANS =                                      \
   CURLOPT_RTSP_SESSION_ID.3                     \
   CURLOPT_RTSP_STREAM_URI.3                     \
   CURLOPT_RTSP_TRANSPORT.3                      \
+  CURLOPT_SASL_AUTHZID.3                        \
   CURLOPT_SASL_IR.3                             \
   CURLOPT_SEEKDATA.3                            \
   CURLOPT_SEEKFUNCTION.3                        \