vtls: add MesaLink to curl_sslbackend enum

MesaLink support was added in commit 57348eb97d but the
backend was never added to the curl_sslbackend enum in curl/curl.h.
This adds the new backend to the enum and updates the relevant docs.

Closes #3195
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

docs/libcurl/curl_global_sslset.3

@@ -42,7 +42,8 @@ typedef enum {
   CURLSSLBACKEND_SCHANNEL = 8,
   CURLSSLBACKEND_DARWINSSL = 9,
   CURLSSLBACKEND_AXTLS = 10,
-  CURLSSLBACKEND_MBEDTLS = 11
+  CURLSSLBACKEND_MBEDTLS = 11,
+  CURLSSLBACKEND_MESALINK = 12
 } curl_sslbackend;
 
 .B "CURLsslset curl_global_sslset(curl_sslbackend " id,

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.3

@@ -56,9 +56,9 @@ The \fIbackend\fP struct member is one of the defines in the CURLSSLBACKEND_*
 series: CURLSSLBACKEND_NONE (when built without TLS support),
 CURLSSLBACKEND_AXTLS, CURLSSLBACKEND_CYASSL, CURLSSLBACKEND_DARWINSSL,
 CURLSSLBACKEND_GNUTLS, CURLSSLBACKEND_GSKIT, CURLSSLBACKEND_MBEDTLS,
-CURLSSLBACKEND_NSS, CURLSSLBACKEND_OPENSSL, CURLSSLBACKEND_POLARSSL or
-CURLSSLBACKEND_SCHANNEL. (Note that the OpenSSL forks are all reported as just
-OpenSSL here.)
+CURLSSLBACKEND_NSS, CURLSSLBACKEND_OPENSSL, CURLSSLBACKEND_POLARSSL,
+CURLSSLBACKEND_SCHANNEL or CURLSSLBACKEND_MESALINK. (Note that the OpenSSL
+forks are all reported as just OpenSSL here.)
 
 The \fIinternals\fP struct member will point to a TLS library specific pointer
 for the active ("in use") SSL connection, with the following underlying types:
@@ -89,6 +89,8 @@ CtxtHandle *
 SSLContext *
 .IP "WolfSSL (formerly CyaSSL)"
 SSL *
+.IP "MesaLink"
+SSL *
 .RE
 
 If the \fIinternals\fP pointer is NULL then either the SSL backend is not

docs/libcurl/symbols-in-versions

@@ -708,6 +708,7 @@ CURLSSLBACKEND_GNUTLS           7.34.0
 CURLSSLBACKEND_GSKIT            7.34.0
 CURLSSLBACKEND_LIBRESSL         7.49.0
 CURLSSLBACKEND_MBEDTLS          7.46.0
+CURLSSLBACKEND_MESALINK         7.62.0
 CURLSSLBACKEND_NONE             7.34.0
 CURLSSLBACKEND_NSS              7.34.0
 CURLSSLBACKEND_OPENSSL          7.34.0