pkcs #10 requests

[Nikos Mavrogiannopoulos]

Following the thread for request to add certain X.509v3 extensions such
as alternative name to certificate requests (pkcs #10 that is), I have
noticed that in PKCS #9 they define a way to insert those extensions as
a PKCS #10 attribute. I've added support for this, thus in the
repository gnutls can generate and parse extensions from certificate
request (CRQ) files. Moreover I have included the additions suggested by
David Marin Carreno to support certain extensions in the CRL files. I'd
appreciate testing and feedback.

regards,
Nikos