x509: add EXFLAG_DUPLICATE and cheap O(1) extension duplicate check

x509: add EXFLAG_DUPLICATE and cheap O(1) extension duplicate check

In ossl_x509v3_cache_extensions(), introduce EXFLAG_DUPLICATE flag to
signal duplicate X.509 extensions. Add O(1) duplicate detection
using a bitset with minimal stack memory footprint, in compliance with
RFC 5280 Section 4.2.

Fixes #26325

Co-authored-by: Tomáš Mráz <tm@t8m.info>
Co-authored-by: David von Oheimb <DDvO@users.noreply.github.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
MergeDate: Tue Mar 17 13:43:13 2026
(Merged from https://github.com/openssl/openssl/pull/30233)