Fix GSSRPC server credential memory leak

Fix GSSRPC server credential memory leak

In svc_auth_gss.c, stop using the global svcauth_gss_creds, and
instead keep a credential in struct svc_rpc_gss_data.  This change
ensures that the same credential is used for each accept_sec_context
call for a particular context, and ensures that the credential is
freed when the authentication data is destroyed.  Also, do not acquire
a credential when the default name is used (as it is in kadmind) as it
is not needed.

Leave the svcauth_gss_creds around for the backportable fix as it is
in the library export list.  It will be removed in a subsequent
commit.

(cherry picked from commit 670d9828086e979d5cdfd26f00ca88958a03754e)

ticket: 8480
version_fixed: 1.13.7