specdb: no duplicate Standard.Stag is allowed

[Jeff Johnson]

There's nothing that really says Standard.Stag has to be unique (per LSB
version), and nothing that enforces it. If the standard tags (which are used
for API table references to lists of referenced specifications) are not unique,
though, docbook will fail the build.

A simple answer seems to be to add something to the schema, like:

  UNIQUE KEY `Stag` (`Stag`),

but that's too simplistic, as the rule is actually not "no duplicates" but "no
duplicates for a given version". There's one current case of usage that fails
the first test but passes the second - the entries with Stag='Gtk', there's one
which was valid for 3.1, withdrawin in 4.0; a second valid for 4.0, and a third
valid since 4.1 (which will end up superseded by yet another entry valid as of
5.0). Since those don't conflict for a single version build, there's no
problem.

It would be nice if there was some way to enforce the rule we do want, though,
as several times there have been sets of standards added which give dupes on
Stag, while the developer has been careful to make sure Sname is not
duplicated.