Refactor rpc_check_path() and wrap it with a macro to allow passing
different policies and permission names.

This allows using the function for non-read operations and simplifies
the message parsing code there.

Also change the stat and list methods to require "list" instead of
"read" permissions which is useful to allow browing the filesystem
without allowing read access to all files.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>