initThumbprints() now takes an application tag argument
so x509 and ssh can coexist.
the thumbprint entries can now hold both sha1 and sha256
hashes. okThumbprint() now takes a len argument for the
hash length used.
the new function okCertificate() hashes the certificate
with both and checks for any matches.
on failure, okCertificate() returns 0 and sets error string.
we also check for include loops now in thumbfiles, limiting
the number of includes to 8.
tlsClient() now can optionally send the server_name in the ClientHello
message by setting the TLSconn.serverName. This is required for some
https sites.
