On unix systems, long enough path names will fail when performing syscalls
like `Lstat`. The current RemoveAll uses several of these syscalls, and so
will fail for long paths. This can be risky, as it can let users "hide"
files from the system or otherwise make long enough paths for programs
to fail. By using `Unlinkat` and `Openat` syscalls instead, RemoveAll is
safer on unix systems. Initially implemented for linux, darwin, and several bsds.

Fixes #27029
Co-authored-by: Giuseppe Capizzi <gcapizzi@pivotal.io>
Co-authored-by: Julia Nedialkova <yulia.nedyalkova@sap.com>

Change-Id: Id9fcdf4775962b021b7ff438dc51ee6d16bb5f56
GitHub-Last-Rev: b30a621fe359fa2acbb055445b54202b0c508167
GitHub-Pull-Request: golang/go#27871
Reviewed-on: https://go-review.googlesource.com/c/137442
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>