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

docs/libcurl/curl_global_init.3

@@ -47,7 +47,7 @@ value unless you are familiar with it and mean to control internal operations
 of libcurl.
 
 This function is thread-safe since libcurl 7.84.0 if
-\fIcurl_version_info(3)\fP has CURL_VERSION_THREADSAFE_INIT feature bit set
+\fIcurl_version_info(3)\fP has CURL_VERSION_THREADSAFE feature bit set
 (most platforms).
 
 If this is not thread-safe, you must not call this function when any other

docs/libcurl/curl_version_info.3

@@ -206,9 +206,10 @@ libcurl was built with support for SSPI. This is only available on Windows and
 makes libcurl use Windows-provided functions for Kerberos, NTLM, SPNEGO and
 Digest authentication. It also allows libcurl to use the current user
 credentials without the app having to pass them on. (Added in 7.13.2)
-.IP CURL_VERSION_THREADSAFE_INIT
+.IP CURL_VERSION_THREADSAFE
 libcurl was built with thread-safety support (Atomic or SRWLOCK) to protect
 curl initialisation. (Added in 7.84.0)
+See \fIlibcurl-thread(3)\fP
 .IP CURL_VERSION_TLSAUTH_SRP
 libcurl was built with support for TLS-SRP (in one or more of the built-in TLS
 backends). (Added in 7.21.4)

docs/libcurl/symbols-in-versions

@@ -172,7 +172,7 @@ CURL_VERSION_PSL                7.47.0
 CURL_VERSION_SPNEGO             7.10.8
 CURL_VERSION_SSL                7.10
 CURL_VERSION_SSPI               7.13.2
-CURL_VERSION_THREADSAFE_INIT    7.84.0
+CURL_VERSION_THREADSAFE         7.84.0
 CURL_VERSION_TLSAUTH_SRP        7.21.4
 CURL_VERSION_UNICODE            7.72.0
 CURL_VERSION_UNIX_SOCKETS       7.40.0