mediafire (login): send referrer so website does not refuse login
mediafire (download, probe): small rewrite to enhance features

- new functions "mediafire_extract_id" and "mediafire_is_file_id"
- fix detection of direct download links
- add error message when file links are given to plowlist or
folder links are given to plowdown

Note: Detection of folder vs. file links is quite fragile. It relies
on folder IDs to consist of exactly 13 digit/letters. This is true
for all links I found but need not be a general fact.
mediafire: add probe support
dl.free.fr (download + probe): upstream modified, fix dead link detection