Add '-ext' option to display extensions in 'x509'

This is to address issue #3932. Support comma-separated string
to specify what extensions to be displayed.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4016)

Remove redundant variable

[to be squashed]