mod_pep_atom_categories: Add a module to index categories in Atom entries
A new node is created for each category, named category-<the category>, which
contains a list of all node/item tuples having said category as a tag.
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summary: STARTTLS failure test
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This module responds to `<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>` with
`<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>`, in order to test
how clients and server behave.
See [RFC6120 Section 5.4.2.2](https://xmpp.org/rfcs/rfc6120.html#rfc.section.5.4.2.2)