OPENSSL_malloc.pod: mention CRYPTO_memdup()

OPENSSL_malloc.pod: mention CRYPTO_memdup()

Add its mentions to NAME, SYNOPSIS, and RETURN VALUES sections.
Also, while at it, put OPENSSL_{str,strn,mem}dup() with the other
OPENSSL_* interfaces, and add mentions of OPENSSL_str{,n}dup()
to RETURN VALUES.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
MergeDate: Wed Apr  8 10:23:55 2026
(Merged from https://github.com/openssl/openssl/pull/30623)

(cherry picked from commit fb6c9d8620eccf38899aeb7bb97223a2ad0f9e00)