Remove the subprocess management functions from xutil.lua, as they were
moved into base/ffi/util.lua, and use them from there.
Also use the cre_storage_size_factor setting when processing credocuments,
to avoid CRE WARNING while indexing too.
Fix dealing with MuPDF document opening failures (previously, these
were not noticed and indexing was retried each time).
poire-z
3c857e4016