Auto-backup optimisation - skip old unavailable unmodified courses

Eloy suggested I file a bug/idea report:

The desire was to expedite backups by skipping over obsolete courses. I define obsolete as a course which is marked as unavailable, and also has not been modified in a month. Since we do weekly backups, a course which is just changed to be unavailable, is still backed up 4 times before it no longer has updated backups. The assumption I make is that the modified date is reliable.

I expect to retain courses from previous semesters for up to a year, and mark them as unavailable as we start a new semester. This keeps them from being listed for their students still registered, yet allows the course to be used for 'import course data' purposes.

Two files are involved, backup/backup_scheduled.php and lang/en/moodle.php (this last to define the word 'skipped' for the backup report. A context-diff relative to 1.5.3+ will be attached.