System cache index cleanup

I have made most system indexes unique.

I have added indexes to match all system caches.

I have renamed some cache names to be clearer.

I have re-ordered the cache entries to be alphabetical.

I have renamed the inheritance *rel columns to be *relid.

I have added a large comment to syscache.c listing steps needed to
install a new system index.

I saw no speed improvement from my changes, but I can imagine cases
where this would be a speedup.

The only thing missing is that I can't seem to get pg_shadow to use an
index from the cache.  When I try it, initdb runs really slow, and the
resulting installation is unusable.  Any ideas anyone?  You can see my
commented-out code in indexing.c and syscache.c.  My guess is that the
strange way we issue pg_exec_query_dest() calls is the cause.  I have no
call to CatalogIndexInsert() for the pg_shadow because of this.  Anyone
want to rewrite user.c to use heap_insert() instead.

initdb required.

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