[krbdev.mit.edu #6787] S4U memory leak

[Arlene Berry via RT]

When using S4U, in src/lib/gssapi/krb5/s4u_gss_glue.c
kg_compose_deleg_cred, it creates a new unique memory credential cache
that is internal to the credential.  It needs to be destroyed when the
credential is released.  However, krb5_gss_release_cred only closes the
cache and it's being leaked.  Under other circumstances the cache field
points to a pre-existing credential cache that should not be destroyed.
There is nothing that indicates whether the cache should be destroyed,
so to solve this we'll add a flag to the credential structure unless you
have a better solution.