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.

ticket: 8480 (new)
target_version: 1.14-next
target_version: 1.13-next