Update posix fs shims to use new API `reject_in_isolation`, which
allows rejection with error code instead of always forcing abort.
Error code chosen for each op is the most appropriate one from the
list in corresponding syscall's manual.

Updated helper APIs to not use quotes (`) around input name while
preparing the message. This allows callers to pass multi-word string
like -- "`read` from stdin".

Change the code to either `EACCES` (if the op is performed on the
path), or `EBADF` (if the op is performed the fd)

Updated ops: `stat`, `opendir`, `ftruncate64`, and `readlink`

Add a new test for fs ops in isolation.

This allows catching extremely incorrect arguments before rejecting
due to isolation.