Update krb5 code to use glue routine lucid context functions
The gssd code should not know about the glue layer's context structure. A previous patch added gss_export_lucid_sec_context() and gss_free_lucid_sec_context() functions to the gssapi glue layer. Use these functions rather than calling directly to the Kerberos gssapi code (which requires the Kerberos context handle rather than the glue's context handle).