datalib: get_my_courses() - add course visibility checks

datalib: get_my_courses() - add course visibility checks

Reworked gmc to perform the course visibility checks. These are
very cheap if $CFG->allowvisiblecoursesinhiddencategories is true.

However, where we have to enforce category visibility, it adds a bit
of work. In simple terms, it adds a DB query to read all the categories,
and extra checks to make sure we are doing the right thing WRT

 - course visibility   vs the permission to see hidden courses
 - category visibility vs the permission to see hidden categories

and still do it quickly.