x/tools/internal/gopathwalk: broken on darwin w/ readdirent: bad file descriptor

[bradfitz]

@randall77, the "syscall: avoiding _getdirentries64 on darwin" CL also seems to have broken x/tools's gopathwalk used by goimports & various other tools:

https://build.golang.org/log/ab1733a9ba43f978acbbd4b7a94d3e4da0477113

Lots of errors, but all involving readdirent: bad file descriptor

[gopherbot]

Change https://golang.org/cl/170698 mentions this issue: syscall: dup the argument to fdopendir

[gopherbot]

Change https://golang.org/cl/170892 mentions this issue: syscall: use openat instead of dup to make a really new file descriptor