Creates a web-accessible URL for a stream to an external or local file.

Compatibility: normal paths and stream wrappers.

There are two kinds of local files:

• "managed files", i.e. those stored by a Drupal-compatible stream wrapper.
  These are files that have either been uploaded by users or were generated
  automatically (for example through CSS aggregation).
• "shipped files", i.e. those outside of the files directory, which ship as
  part of Drupal core or contributed modules or themes.