Improve the get_my_courses() behaviour

Current get_my_courses() implementation can perform a bit better caching:

1) Modify get_my_courses to ALWAYS fetch (an cache) ALL the user courses and return from cache respecting limit. -> Yu
2) Modify the my/index.php call to add the modinfo field (won't use the cache, obviously) -> Eloy
3) Modify as many get_my_courses() calls to use the default fields (because they are cached) -> Eloy