[hooks/security] provide attribute "add" permission
As of today, the update permission on attributes is checked at entity
*creation* time. This forbids using update permissions the proper way.
We set it to be checked at entity update time only.
We introduce a specific 'add' permission rule for attributes.
For backward compatibility, its default value will be the same as the
current 'update' permission.
Notes:
* needs a new yams version (ticket #149216)
* introduces two new 'add_permissions' rdefs (attribute - group|rqlexpr)
* if the update permission was () and the bw compat kicks in, the rule
is not enforced, to avoid un-creatable entity types -- this
restriction will be lifted when the bw compat is gone
* small internal refactoring on check_entity_attributes
* one small pre 3.6.1 bw compat snippet must be removed from schemaserial
Closes #2965518.