Add support for extracting subjectAltName entries of type
rfc822Name and dNSName into SSL_{CLIENT,SERVER}_SAN_{Email,DNS}_n
variables.

* docs/manual/mod/mod_ssl.xml: add SSL_*_SAN_*_n entries to the
  environment variables table

* modules/ssl/ssl_engine_kernel.c: in ssl_hook_Fixup, add extraction
  of subjectAltName entries for the "StdEnvVars" case

* modules/ssl/ssl_engine_vars.c: add support for retrieving the
  SSL_{CLIENT,SERVER}_SAN_{Email,DNS}_n variables, either with
  individual on-demand lookup (ssl_var_lookup_ssl_cert_san),
  or with full-list extraction to the environment ("StdEnvVars")

* modules/ssl/ssl_private.h: add modssl_var_extract_san_entries prototype

* modules/ssl/ssl_util_ssl.c: implement SSL_X509_getSAN and
  SSL_ASN1_STRING_to_utf8 helper functions, with factoring out common
  code from SSL_X509_getIDs and SSL_X509_NAME_ENTRY_to_string where
  suitable. Limit SSL_X509_getSAN to the two most common subjectAltName
  entry types appearing in user or serv...