Mimic Key Usage and Basic Constraints

Mimic Key Usage and Basic Constraints

There are cases where the generated certificates do not mimic enough properties
and secure connection with the client fails. For example, Squid does not mimic
Key Usage extensions. Clients using GnuTLS (or similar libraries that validate
server certificate using those extensions) fail to secure the connection with
Squid.

This patch add mimicking for the following extensions, which are considered
as safe to mimic:
    * X509v3 Key Usage
    * X509v3 Extended Key Usage,
    * X509v3 Basic Constraints CA.

We would be happy to add more "safe to mimic" extensions if users request (and
vouch for) them.

This is a Measurement Factory project