gss: implement gss_krb5_ccache_name()

Correctly implement gss_krb5_ccache_name() in terms of
gss_set_sec_context_option(GSS_KRB5_CCACHE_NAME_X). The previous implementation
was a NOOP.

Note: global ccache name should really be thread-specific rather than global.

Closes #803.

lib/gssapi/krb5/gsskrb5_locl.h

@@ -145,4 +145,9 @@ extern HEIMDAL_MUTEX gssapi_keytab_mutex;
 #define SC_ORDER          0x0080
 #define SC_AUTHENTICATOR  0x0100
 
+struct gsskrb5_ccache_name_args {
+    const char *name;
+    const char *out_name;
+};
+
 #endif