Fix issue #6269

Added isLocal() method to Storage to find out whether the storage is
local or not.
This method is used for the x-sendfile logic to find out whether to add
the headers.

close statement was never executed due to it being after a return
statement.

This reverts commit 5a5b6f18.

init_var and init_radio  where only used in the installation template

A new unit test has been added in consequence.

Whenever an exception occurs in the sabre connector code or code called
by it, it will be logged.

This plugin approach is needed because Sabre already catches exceptions
to return them to the client in the XML response, so they don't appear
logged in the web server log.

This will make it much easier to debug syncing issues.

The Freebox is the multimedia device of a french Internet provider: Free. This device provides a seedbox which uses the user agent "Mozilla/5.0". In the "Content-Disposition" header, if the "filename" key is used with the "filename*=UTF-8''" value, the seedbox does not take care about the header and saves the file name with the origin URL. This patch brings the support for the Freebox users.

Sometimes it's not possible to disable APC entirely and some of
apc_functions are disabled. Only thing which is possible is
to disable apc.enable option.