apps x509: passing PKCS#11 URL as -signkey

apps x509: passing PKCS#11 URL as -signkey

OpenSSL 1.1.0 has extended option checking, and rejects passing a PKCS#11
engine URL to "-signkey" option. The actual code is ready to take it.

Change the option parsing to allow an engine URL to be passed and modify
the manpage accordingly.

CLA: trivial

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11086)