Rename and collapse tls12_get_sigandhash_cbb().

Rename and collapse tls12_get_sigandhash_cbb().

Now that all callers of tls12_get_sigandhash() have been converted to CBB,
collapse tls12_get_sigandhash() and tls12_get_sigandhash_cbb() into a
single function. Rename it to tls12_gethashandsig() to be representative
of the actual order of the sigalgs parameters, and perform some other
clean up.

ok inoguchi@ tb@