Use get_file_url instead of manual slasharguments check

To fix MDL-13792, the function get_file_url was added. This bug aims to make use of it in every possible place. I will send in a big patch later (starting with 1.8) for review.

There is a problem with links to rss files, as most of them are to /rss/file.php, not /file.php. See /blog/rsslib.php, line 41, for one case of this. Possibilities :
1) don't do anything, xml files shouldn't have non-ascii file names anyway
2) modify get_file_url to make a link to /rss/file.php in these cases
3) make a different function for rss links

I vote for #2. Petr? Eloy? What do you think?