Disable cert validation when pre-establishing connections in rlm_rest

We don't have a valid TLS configuration to use.

libcurl appears to do the right thing, and validate the cert when reconfigured to for subsequent requests.

We tell libcurl to *only* establish the connection and not send any data, so there's no chance of us
leaking sensitive information.
Print out error message returned by the server
Fix issue in configured 'forced' types
Allow 'plain' as a valid WEB API type