hsts: add support for Strict-Transport-Security

- enable in the build (configure)
- header parsing
- host name lookup
- unit tests for the above
- CI build
- CURL_VERSION_HSTS bit
- curl_version_info support
- curl -V output
- curl-config --features
- CURLOPT_HSTS_CTRL
- man page for CURLOPT_HSTS_CTRL
- curl --hsts (sets CURLOPT_HSTS_CTRL and works with --libcurl)
- man page for --hsts
- save cache to disk
- load cache from disk
- CURLOPT_HSTS
- man page for CURLOPT_HSTS
- added docs/HSTS.md
- fixed --version docs
- adjusted curl_easy_duphandle

Closes #5896

src/tool_setopt.c

@@ -62,6 +62,11 @@ const struct NameValue setopt_nv_CURL_SOCKS_PROXY[] = {
   NVEND,
 };
 
+const struct NameValueUnsigned setopt_nv_CURLHSTS[] = {
+  NV(CURLHSTS_ENABLE),
+  NVEND,
+};
+
 const struct NameValueUnsigned setopt_nv_CURLAUTH[] = {
   NV(CURLAUTH_ANY),             /* combination */
   NV(CURLAUTH_ANYSAFE),         /* combination */