Fix openssl req with -addext subjectAltName=dirName

Fix openssl req with -addext subjectAltName=dirName

The syntax check of the -addext fails because the
X509V3_CTX is used to lookup the referenced section,
but the wrong configuration file is used, where only
a default section with all passed in -addext lines is available.
Thus it was not possible to use the subjectAltName=dirName:section
as an -addext parameter.  Probably other extensions as well.

This change affects only the syntax check, the real extension
was already created with correct parameters.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23669)