BUILD: ssl: make X509_NAME usage OpenSSL 4.0 ready

BUILD: ssl: make X509_NAME usage OpenSSL 4.0 ready

Starting with OpenSSL 4.0, X509_get_subject_name(), X509_get_issuer_name(),
and X509_CRL_get_issuer() return a const-qualified X509_NAME pointer.
Similarly, X509_NAME_get_entry() returns a const X509_NAME_ENTRY *, and
X509_NAME_ENTRY_get_data() returns a const ASN1_STRING *.

Introduce the __X509_NAME_CONST__ macro (defined to 'const' for OpenSSL
>= 4.0.0, empty for WolfSSL and older OpenSSL version which lacks const
on these APIs) and use it to qualify X509_NAME * variables and the
parameters of the three DN helper functions ssl_sock_get_dn_entry(),
ssl_sock_get_dn_formatted(), and ssl_sock_get_dn_oneline(). This avoids
both const-qualifier warnings on OpenSSL 4.0 and discarded-qualifier
warnings on WolfSSL, without needing explicit casts at call sites.

In ssl_sock.c (ssl_get_client_ca_file) and ssl_gencert.c
(ssl_sock_do_create_cert), a __X509_NAME_CONST__ X509_NAME * variable was
being reused to store the result of X509_NAME_dup() and then passed to
mutating functions (X509_NAME_add_entry_by_txt, X509_NAME_free). Introduce
separate X509_NAME * variables (xn_dup, subject) to hold the mutable
duplicate.

Original patch from Alexandr Nedvedicky <sashan@openssl.org>:
https://www.mail-archive.com/haproxy@formilux.org/msg46696.html