addresses #521
mergerfs is written using libfuse's high level API. The HLAPI forces a certain
behavior with regard to unlinking or rename overwriting of open files. It
renames the file to .fuse_hiddenXXXXXX and when incoming requests refer
to the unlinked file it will use path based calls and use that path.
This behavior leads to rmdirs failing or some software noticing
the .fuse_hidden files.
This patch changes the HLAPI and removes entirely the .fuse_hidden behavior.
See the README for more.
Parsing of and acting upon "-o threads" option is done in the bundled `libfuse/`,
so move the corresponding help text there too.
This make help text consistent with behavior
when building against system libfuse,
which doesn't support this option.
