Add --disable-openssl-vercheck configure option

Add "--disable-openssl-vercheck" configure option, which removes
checking for vulnerable OpenSSL versions. It is supposed to be used by
downstream packagers and distributions who have other means to ensure
vulnerabilities are fixed, such as versioned package dependencies and
vulnerability handling processes.

This avoids the necessity of editing radiusd.conf on package upgrade to
make sure it keeps working. At the same time, it provides safe default
to those installing FreeRADIUS from source.

This would allow us (Red Hat) to upgrade FreeRADIUS from 3.0.1 smoothly.
Otherwise we would need to provide instructions for users to edit their
radiusd.conf to confirm, essentially, that the job they're paying us to do
(keeping their systems secure) is done. This might be a problem to other
distributions as well.

I understand that this patch might not be ideal. Please tell me if there is
any problem with it and I'll fix it. Well, if it isn't totally unacceptable,
of course. Thank you.