Add KRB5RCACHENAME and default_rcache_name

Add KRB5RCACHENAME and default_rcache_name

In krb5_get_server_rcache(), stop constructing a residual value based
on piece and the euid, and instead resolve default replay cache
(previously an unused operation, as krb5_rc_default() was not part of
the API and was never used).

To determine the default replay cache name, try the KRB5RCACHENAME
environment variable first, and then try KRB5RCACHETYPE for
compatibility.  If neither of those environment variables are set, try
the default_rcache_name profile relation in [libdefaults] before
falling back to "dfl:".

ticket: 8786