mod_ssl: Expose serialNumber (2.5.4.5) as standard env var from certificate DN

serialNumber (2.5.4.5) is a standard LDAP attribute embedded in the subject's
and/or issuer's DN, extract it by standard means from the DN and expose
via StdEnvVars.

PR: 35154
Co-authored-by: Michael Osipov <michaelo@apache.org>
Submitted by: Benjamin Demarteau <benjamin.demarteau@liege.be>
Github: closes #644


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1933908 13f79535-47bb-0310-9956-ffa450edef68

Author: notroj

changes-entries/ssl-serialnumber.txt

@@ -0,0 +1,4 @@
+  *) mod_ssl: Add SerialNumber as a recognized attribute type for SSL
+     distinguished name variables.  [Michael Osipov <michaelo apache.org>,
+     Benjamin Demarteau <benjamin.demarteau liege.be>]
+

modules/ssl/ssl_engine_vars.c

@@ -720,6 +720,7 @@ static const struct {
     { "G",     NID_givenName,              1 },
     { "S",     NID_surname,                1 },
     { "D",     NID_description,            1 },
+    { "SerialNumber", NID_serialNumber,    1 },
 #ifdef NID_userId
     { "UID",   NID_userId,                 1 },
 #endif