gnutls: Support CURLOPT_KEYPASSWD

The gnutls vtls back-end was previously ignoring any password set via
CURLOPT_KEYPASSWD. Presumably this was because
gnutls_certificate_set_x509_key_file did not support encrypted keys.

gnutls now has a gnutls_certificate_set_x509_key_file2 function that
does support encrypted keys. Let's determine at compile time whether the
available gnutls supports this new function. If it does then use it to
pass the password. If it does not then emit a helpful diagnostic if a
password is set. This is preferable to the previous behaviour of just
failing to read the certificate without giving a reason in that case.

Signed-off-by: Mike Crowe <mac@mcrowe.com>