Do not have the module represent themselves with the
gfsec_scheme_module_t structure. Now a module must just take care
of providing the appropriate functions (currently only "get_file")
to perform the task.
The initialization function in scheme-module will assemble the
structures and call any initialization function required.
This patch introduces the notion of a "scheme module". Such a
module provides a "get_file" function to retrieve the contents of
a file located at a specified URI.
Currently available modules:
- the "file" module, supporting the "file://" URI scheme;
- the "libmtp" module, using the LibMTP library to support the
- the "gio" module, using the GLib-GIO library to support the
"uuid://" and "label://" schemes.