Drop support for custom meta-attributes at a major or minor version bump.

At that time, we can also use the schema to restrict meta-attribute names to those that are supported by Pacemaker. This will prevent simple mistakes like misspelling a meta-attribute name.

This could be a simple enumeration in the schema, or we could move to a more structured syntax (keeping the ability to use rules). For example:

<primitive id="rsc1" ...>
  <options id="rsc1-options1" priority="10"/>
  <options id="rsc1-options2" is-managed="false">
    <rule .../>
  </options>
</primitive>