ryushare: add probe support
netload_in: add probe support

This uses the official API which requires an key. For now, a
fixed key is used although I'm not sure whether this is intended.

If problems arise we'll have to add an account requirement, so
each account's own API key can be retrived and used.
depositfiles (upload): change default upload url (depositfiles.com => dfiles.eu)
megashares: add probe support
netload_in (probe): fix alternate link format