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
a94d6fe706