Backport from HEAD: add "SSLUserName" directive to set r->user based

Backport from HEAD: add "SSLUserName" directive to set r->user based
on a chosen SSL environment variable name.

* modules/ssl/mod_ssl.h (struct SSLDirConfigRec): Add
szUserName field.

* modules/ssl/ssl_engine_config.c (ssl_config_perdir_create,
ssl_config_perdir_merge): Initialize and merge szUserName field.
(ssl_cmd_SSLUserName): New function.

* modules/ssl/ssl_engine_kernel.c (ssl_hook_Fixup): Set r->user to
the value of the chosen SSL environment variable.

* modules/ssl/mod_ssl.c: Add SSLUserName config directive.

PR: 20957
Submitted by: Martin v. Loewis
Reviewed by: trawick, jorton, nd

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@104770 13f79535-47bb-0310-9956-ffa450edef68