File scheme providers uses non POSIX compatible options

[lambdalisue]

In most cases, this is not a matter but the file scheme uses the following non POSIX compatible options thus fern might not work on the really old machine even Vim 8.1 works on such platforms (e.g. #237 )

find

-maxdepth and -follow are not POSIX compatible.
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html

ls

-U is not POSIX compatible
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ls.html