Arch: Mark deleted packages as dead and hide them from listings

We've got a lot of packages in the DB that have long been removed from
the Arch repos. These are still indexed, but won't clutter the package
listing anymore.

Also fixed an issue with packages.id numbers getting rather large
because the indexer allocates a new ID for every package on every
update.

sql/schema.sql

@@ -19,8 +19,15 @@ CREATE TABLE contents (
 CREATE TABLE packages (
   id       SERIAL    PRIMARY KEY,
   system   integer   NOT NULL REFERENCES systems(id) ON DELETE CASCADE ON UPDATE CASCADE,
-  category varchar,
+  category varchar   NOT NULL,
   name     varchar   NOT NULL,
+  -- Whether this package has been seen in the last repository update. This
+  -- field is only updated for a few systems that are likely to delete packages
+  -- over time; non-rolling-release distros tend to not delete packages after
+  -- all.
+  -- Packages where the latest version does not have any man pages may also be
+  -- marked as dead even if the package is still available in the repos.
+  dead     boolean   NOT NULL DEFAULT FALSE,
   UNIQUE(system, name, category) -- Note the order, lookups on (system,name) are common
 );