version: rename threadsafe-init to threadsafe

Referring to Daniel's article [1], making the init function thread-safe
was the last bit to make libcurl thread-safe as a whole. So the name of
the feature may as well be the more concise 'threadsafe', also telling
the story that libcurl is now fully thread-safe, not just its init
function. Chances are high that libcurl wants to remain so in the
future, so there is little likelihood of ever needing any other distinct
`threadsafe-<name>` feature flags.

For consistency we also shorten `CURL_VERSION_THREADSAFE_INIT` to
`CURL_VERSION_THREADSAFE`, update its description and reference libcurl's
thread safety documentation.

[1]: https://daniel.haxx.se/blog/2022/06/08/making-libcurl-init-more-thread-safe/

Reviewed-by: Daniel Stenberg
Reviewed-by: Jay Satiro
Closes #8989

src/tool_help.c

@@ -112,7 +112,7 @@ static const struct feat feats[] = {
   {"alt-svc",        CURL_VERSION_ALTSVC},
   {"HSTS",           CURL_VERSION_HSTS},
   {"gsasl",          CURL_VERSION_GSASL},
-  {"threadsafe-init",CURL_VERSION_THREADSAFE_INIT},
+  {"threadsafe",     CURL_VERSION_THREADSAFE},
 };
 
 static void print_category(curlhelp_t category)