This changes file descriptors from uint32 to int32 and the
corresponding file table to reject negative values. This ensures
invalid values aren't mistaken for very large descriptor entries, which
can use a lot of memory as the table implementation isn't designed to
be sparse.
See https://pubs.opengroup.org/onlinepubs/9699919799/functions/dirfd.html#tag_16_90
Signed-off-by: Adrian Cole <adrian@tetrate.io>
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