Allow extraction of the values of SSL certificate extensions into

Allow extraction of the values of SSL certificate extensions into
environment variables, so that their value can be used by any
module that is aware of environment variables, as in:

  SetEnvIf OID("2.16.840.1.113730.1.13") "(.*) Generated (Certificate)" ca=$1
sets
  ca=TinyCA
if the cert was issued by TinyCA.

Similarly,
  SetenvIf OID("2.16.840.1.113730.1.13") "(.*)" NetscapeComment=$1
will set $NetscapeComment to the whole string.

It is technically allowed to have multiple instances of an extension
field, all with the same oid. In this case, the environment variable
will be set to the list of all fields, separated by commas.

The [PATCH] uses a cross-module call from mod_setenvif to
mod_ssl (the latter may also be missing: in this case the
variable will never be set). It calls a common function
in the ssl module that is also used for the SSLRequire
directive's test.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@220307 13f79535-47bb-0310-9956-ffa450edef68