Implement man selection algorithm in SQL + fix various related bugs

Man selection has to be performed over several thousand rows in some
cases. Loading all those in Perl and then doing the selection isn't very
efficient[1]. The getman() implementation was also buggy: The comparison
function used to determine which man page should be preferred was not
associative[2], and the result thus depended on the order in which the
man pages were compared. This resulted in some wrong selections in some
cases.

While I was at it, I also made the selection more strict:
- /man/unknown-hash would previously ignore the hash and just select
  whatever man page. Now it results in a 404.
- Same with /man.unknown-section
- /man.section/hash is now disallowed, it's either /man.section or
  /man/hash.

1) Note that all possible man pages are currently still loaded into Perl
anyway, because the ugly navigation menu on the right needs them. I plan
to revamp that entire menu to be more efficient and usable.

2) Initially I wrote the SQL implementation in a similar fashion to the
Perl implementation, and ended up with the same bug. I wasted more than
a day before I finally got to the current CTE query.

sql/schema.sql

@@ -171,3 +171,13 @@ $$ LANGUAGE SQL;
 CREATE OR REPLACE FUNCTION name_from_filename(text) RETURNS text AS $$
   SELECT regexp_replace(basename_from_filename($1), E'^(.+)\\.[^.]+$', E'\\1');
 $$ LANGUAGE SQL;
+
+
+CREATE OR REPLACE FUNCTION is_english_locale(locale text) RETURNS bool AS $$
+  SELECT locale IS NULL OR locale LIKE 'en%';
+$$ IMMUTABLE LANGUAGE SQL;
+
+
+CREATE OR REPLACE FUNCTION is_standard_man_location(path text) RETURNS bool AS $$
+  SELECT path LIKE '/usr/share/man/man%' OR path LIKE '/usr/local/man/man%';
+$$ IMMUTABLE LANGUAGE sql;