prevent directory traversals below _base_ (fixes #2):
- unify read and size into Fs_common
- introduce check_filename which uses realpath to ensure that requested name is below base
- guard all file system accesses with check_filename
use Lwt_unit.LargeFile for stat
use OCaml code instead of interfacing realpath
remove c wrapper; regenerate oasis
fix normalisation
reindent, as suggested by @dsheets
simplify
make split_string tail-recursive
Prepare for 1.1.1 release; regen oasis