certificate: Use SHA256 fingerprints instead of SHA1

This meant adding a get_fingerprint_sha256 function to the certificate scheme
structs, which meant adding a struct_size member because we ran out of reserved
members there.

The API-facing purple_certificate_get_fingerprint_sha256() has a fallback
parameter to use sha1 if the SSL plugin doesn't implement this function
(probably an outdated installation, or a third party SSL plugin). When using
the function for display purposes, the fallback is disabled and it returns
NULL, but when using it to compare certificates it's better to have at least
the SHA1.

In functions like purple_certificate_display_x509(), some slight changes to
translatable strings would have been required. Since we're in a string freeze
right now, I avoided those by concatenating a language-neutral "SHA256: %s" at
the end of those messages. The SHA1 line used the word "fingerprint" but we
can't reuse that translation. This should be cleaned up after the release.