Allow file server items to re-map URI paths to a different path

[Nicklas Nordborg]

A file item that is a reference to an external file define an absolute URI in the form:

schema://host/path

When the file item is linked with a FileServer item the file server replaces the host part with the host and port defined by the file server. Eg.

http://host.org/doc.txt could be re-mapped to http://otherhost.org/doc.txt

In some cases it would be useful to also be able to re-map the path part. For example if someone wants to implement a file:// or sftp:// protocol handler it would not be advisable to be able to get *any* file on the server by saying file:///path/to/sensitive/data.

It would be useful if the file server item defines a 'safe' root path that is automatically added to the file URI. Eg.

file:///path/to/data is re-mapped to file:///safe-root/path/to/data

In this case we must also make sure that the URI doesn't contain '..' to move upwards in the directory structure thereby bypassing the safe root path.

[Nicklas Nordborg]

(In [6488]) Fixes #1816: Allow file server items to re-map URI paths to a different path