Until Moodle 2.0, the sql_substr() helper function (dmllib), is used without parameters at simply returns the "substring" function used for each DB.

Change this behaviour so it will look like:

$DB->sql_substr(string, from, len)

with len being optional.

- Implement it in new moodle_databases.
- Document it.
- Look for wrong usages in core.
- Add some tests, with and without len.

Ciao :-)