Return false in case the certificate is about to expire, causing the
(only) caller to subsequently verify the proxy.

The mode passed to access is an int and not a string, like in fopen.

Just rely on the canonical loop over the error stack, without special
treatment for an expired certificate, which is caught somewhere else in
any case.
Be sure that the data added to an error is a text string before printing
it.

The code mentions OpenSSL functions related to error management, which
has changed in OpenSSL 3, deprecating old practices.

The use of the function name in errors has been abandoned in OpenSSL 3.
Profit to replace the use of sprintf with snprintf.

To be more compatible with how it was done before.