Honor SSL_CERT_DIR & SSL_CERT_FILE env vars

The Perl IO::Socket::SSL keeps us from benefitting from OpenSSL defaults
initialisation for CA certificates location.  Before we ask the OpenSSL
binary for locations, we now do the same thing that the library init
would do, and preferentially honour `$SSL_CERT_DIR` and then
`$SSL_CERT_FILE` as locations.

I've also corrected the POD documentation on this topic, which was out
of date.