libfido2: tweak credential to read fido2 PIN from

libfido2: tweak credential to read fido2 PIN from

Querying a fido2 PIN via askpw for enrolling is currently used in two
places: cryptenroll and homectl. So far we sloppily used the same fixed
credential name "fido2-pin" in both cases. Let's tweak that and make the
credential name match the other credentials cryptenroll or home query,
i.e. using the cryptenroll.* and home.* namespaces.

This is particular done in light of #31370, which wants to make the
credential name public. We really should get the name in order before
making it public.