Change gnutls certificate_file processing and interactive prompts.
Accept a saved cert at any point in the chain, except for those
revoked or with invalid dates.
The INSECUREALG processing was a bit goofed up. It was skipping the
prompt for every chain except the host, and was only displaying an
error for entry 1 (the host being 0, root being len-1). Instead
prompt at any point, and display a warning as with the other errors.
Add a CERTERR_OTHER type to capture other bits. Recently GnuTLS has
added additional certificate_status_t types. We may want to improve
the prompts in the future for those, but for now at least make sure
it's recorded in certerr.