Change the magic number used to introduce a trie file, so that instead

Change the magic number used to introduce a trie file, so that instead
of a single unsigned long it's a big structure filled with various
integer and pointer types. The idea is that when we check it on
reading an existing index file, it should point out any ABI mismatches
between the platform which generated the index and the one reading it.

In the course of this work I've had to actually add code to check the
magic numbers, which previously weren't checked at all and so would
have been useful only to file(1).

git-svn-id: svn://svn.tartarus.org/sgt/agedu@9723 cda61777-01e9-0310-a592-d414129be87e