Improve gss_store_cred() behavior

Improve gss_store_cred() behavior

Select an output credential cache using similar logic to kinit.  Do
not require the target cache to be initialized.

Try to use the per-thread cache set by gss_krb5_ccache_name() if no
output cache was specified via a cred store.

When the destination is a collection, honor the default_cred flag by
switching the primary cache to the selected output cache.  When the
destination is not a collection, ignore the default_cred flag.
(Previously the default_cred flag was mandatory for gss_store_cred()
even though it is an advisory flag, and ignored for
gss_store_cred_into() even if no ccache was specified in the cred
store.)

Honor the overwrite_cred flag by refusing to replace an initialized
cache if it is not set.  Stop using gss_acquire_cred() for this
purpose as it could go out and fetch credentials from a client keytab.

Perform atomic replacement of the target cache when possible, using
krb5_cc_move().

Add a test harness for calling gss_store_cred() or
gss_store_cred_into() and a suite of tests.  Fix a broken trace log
message for krb5_cc_move() and update the expected trace logs for an
existing t_credstore.py test.

ticket: 8010