list: rename internal fields
list: rename internal fields

This makes conversion to using list.h easier because the compiler will error
on an unknown field name rather than accepting `head` and `tail` and missing
some changes.

Reviewed-by: Tim Hudson <eb0be8e447f673b41b10eda09d63cd72526b0a42@openssl.org>
Reviewed-by: Shane Lontis <452ce16516ceed26291fe7de3f8b53540d83864e@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/19377)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl