largefiles: replace invocation of os.path module by vfs in lfutil.py
Replaces invocations os.path functions to methods in vfs. Unfortunately
(in my view) this makes code less readable, because instead of using
clear variable names with path it needs to replace them with vfs(..).
I need guidance how to make such transition look more readable.
For example in this patch there is example with few places with
wvfs.join(standindir), standindir before this patch was absolute
path, in this it is changed to relative because it is used also
in expression wvfs.join(standindir, pat).