gssapi: credential store extensions (#451)

Implement the GSS-API credential store API extensions defined by MIT here:

https://k5wiki.kerberos.org/wiki/Projects/Credential_Store_extensions

Note: we kill off gss_acquire_cred_ext() here. This was never a public API,
although mechanisms could have implemented it and I briefly used it in my
BrowserID prototype mechanism. gss_acquire_cred_ext_from() occupies the place
in the dispatch table where gss_acquire_cred_ext() used to, but this structure
was never visible outside Heimdal (i.e. it is only used by internal
mechanisms);

(Mechanisms that need to accept arbitrary key/value dictionaries from
applications should now implement gss_acquire_cred_from().)

lib/gssapi/netlogon/external.c

@@ -80,7 +80,7 @@ static gssapi_mech_interface_desc netlogon_mech = {
     NULL, /* gm_store_cred */
     NULL, /* gm_export_cred */
     NULL, /* gm_import_cred */
-    NULL, /* gm_acquire_cred_ext */
+    NULL, /* gm_acquire_cred_from */
     NULL, /* gm_iter_creds */
     NULL, /* gm_destroy_cred */
     NULL, /* gm_cred_hold */
@@ -98,6 +98,8 @@ static gssapi_mech_interface_desc netlogon_mech = {
     NULL, /* gm_delete_name_attribute */
     NULL, /* gm_export_name_composite */
     NULL, /* gm_duplicate_cred */
+    NULL, /* gm_add_cred_from */
+    NULL, /* gm_store_cred_into */
     NULL  /* gm_compat */
 };