Clean up thread_local function names in initthread.c

Clean up thread_local function names in initthread.c

The naming of the thread-local management functions in initthread.c is a
bit...much.  init_manage_thread_local/init_fetch_clear_thread_local, etc
are better written as just manage_thread_local/clear_thread_local/etc.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)