Remove direct mysqli use

[pgeorgi]

Generic DB rework:

• replaces mysqli use with PDO hardcoded to mysql. The idea is to eventually support other DBMS, too.
• unify database schema handling in system/includes/db_updates.php. See commit message for details.

There are some more bits hidden in there: removed some gettype() calls (replaced with is_string, as recommended by the docs), start to use prepared statements and bound parameters, instead of manual string escaping.

[pgeorgi]

The scope of this pull request grew a "bit". Given its scope, I'd rather not simply push it to upstream/master, but have some feedback on this...

The advantage in terms of DBMS portability is that all schema handling (which really is where the differences in notation show) is now centralized in that file. Future work could concentrate on that. Generally speaking the advantage is that this has an easier code flow. before there were some hacks with "only run 100 updates" and the like, presumably (from the comments) to avoid and endless loop. That loop is no more.