tpm2-util: rename tpm2_get_pin_auth() → tpm2_auth_value_from_pin()

tpm2-util: rename tpm2_get_pin_auth() → tpm2_auth_value_from_pin()

Just some renaming. I found the old name a bit confusing since it sounds
as if this would get the pin from somewhere, but it really doesn't. It
just converts a PIN into an auth_value, and I think saying so explicitly
makes things easier to grok.