Fix GSSAPI krb5 cred ccache import

Fix GSSAPI krb5 cred ccache import

json_to_ccache was incorrectly indexing the JSON array when restoring
a memory ccache.  Fix it.

Add test coverage for a multi-cred ccache by exporting/importing the
synthesized S4U2Proxy delegated cred in t_s4u2proxy_krb5.c; move
export_import_cred from t_export_cred.c to common.c to facilitate
this.  Make a note in t_export_cred.py that this case is covered in
t_s4u.py.

(cherry picked from commit 48dd01f29b893a958a64dcf6eb0b734e8463425b)

ticket: 7723 (new)
version_fixed: 1.11.4
status: resolved