stream_lavf: escape disallowed characters in http URLs

In my opinion, libavformat should be doing this. But a patch handling a
very safe case rejected, so I suppose we have to do it manually. (This
patch was only escaping spaces, which can never work because they break
the basic syntax of the HTTP protocol.)

This commit attempts to do 2 things:
- Try to guess whether libavformat will use the URL for http. This is
  not always trivial, because some protocols will recursively pass part
  of the user URL to http in some way.
- Try to fix invalid URLs. We fix only the simplest case: only
  characters that are never valid are escaped. This excludes invalid
  escape codes, which happen with freestanding '%' characters.

Fixes #1495.