TLS: add BearSSL vtls implementation

Closes #4597

docs/FAQ

@@ -447,9 +447,9 @@ FAQ
 
   curl can be built to use one of the following SSL alternatives: OpenSSL,
   libressl, BoringSSL, GnuTLS, wolfSSL, NSS, mbedTLS, MesaLink, Secure
-  Transport (native iOS/OS X), Schannel (native Windows) or GSKit (native IBM
-  i). They all have their pros and cons, and we try to maintain a comparison
-  of them here: https://curl.haxx.se/docs/ssl-compared.html
+  Transport (native iOS/OS X), Schannel (native Windows), GSKit (native IBM
+  i), or BearSSL. They all have their pros and cons, and we try to maintain a
+  comparison of them here: https://curl.haxx.se/docs/ssl-compared.html
 
   2.3 Where can I find a copy of LIBEAY32.DLL?
 

docs/INSTALL.md

@@ -120,6 +120,7 @@ libressl.
  - schannel: `--without-ssl --with-schannel`
  - secure transport: `--without-ssl --with-secure-transport`
  - MesaLink: `--without-ssl --with-mesalink`
+ - BearSSL: `--without-ssl --with-bearssl`
 
 # Windows
 

docs/LICENSE-MIXING.md

@@ -75,6 +75,11 @@ not have the announcement clause that collides with GPL.
  (May be used for SSL/TLS support) As an OpenSSL fork, it has the same
  license as that.
 
+## BearSSL
+
+ (May be used for SSL/TLS support) Uses an MIT license that is very liberal
+ and imposes no restrictions on any other library or part you may link with.
+
 ## c-ares
 
  (Used for asynchronous name resolves) Uses an MIT license that is very

docs/libcurl/curl_global_sslset.3

@@ -43,7 +43,8 @@ typedef enum {
   CURLSSLBACKEND_DARWINSSL = 9,
   CURLSSLBACKEND_AXTLS = 10, /* deprecated */
   CURLSSLBACKEND_MBEDTLS = 11,
-  CURLSSLBACKEND_MESALINK = 12
+  CURLSSLBACKEND_MESALINK = 12,
+  CURLSSLBACKEND_BEARSSL = 13
 } curl_sslbackend;
 
 .B "CURLsslset curl_global_sslset(curl_sslbackend " id,

docs/libcurl/symbols-in-versions

@@ -720,6 +720,7 @@ CURLSSH_AUTH_NONE               7.16.1
 CURLSSH_AUTH_PASSWORD           7.16.1
 CURLSSH_AUTH_PUBLICKEY          7.16.1
 CURLSSLBACKEND_AXTLS            7.38.0       7.61.0
+CURLSSLBACKEND_BEARSSL          7.68.0
 CURLSSLBACKEND_BORINGSSL        7.49.0
 CURLSSLBACKEND_CYASSL           7.34.0
 CURLSSLBACKEND_DARWINSSL        7.34.0       7.64.1