v0.4.0

This release primarily includes a few minor breaking changes to make the
MkdirAll and SecureJoin interfaces more robust against accidental
misuse.

• SecureJoin(VFS) will now return an error if the provided root is not a
  filepath.Clean'd path.

  While it is ultimately the responsibility of the caller to ensure the root is
  a safe path to use, passing a path like /symlink/.. as a root would result
  in the SecureJoin'd path being placed in / even though /symlink/..
  might be a different directory, and so we should more strongly discourage
  such usage.

  All major users of securejoin.SecureJoin already ensure that the paths they
  provide are safe (and this is ultimately a question of user error), but
  removing this foot-gun is probably a good idea. Of course, this is
  necessarily a breaking API change (though we expect no real users to be
  affected by it).

  Thanks to Erik Sjölund, who initially
  reported this issue as a possible security issue.

• MkdirAll and MkdirHandle now take an os.FileMode-style mode argument
  instead of a raw unix.S_*-style mode argument, which may cause compile-time
  type errors depending on how you use filepath-securejoin. For most users,
  there will be no change in behaviour aside from the type change (as the
  bottom 0o777 bits are the same in both formats, and most users are probably
  only using those bits).

  However, if you were using unix.S_ISVTX to set the sticky bit with
  MkdirAll(Handle) you will need to switch to os.ModeSticky otherwise you
  will get a runtime error with this update. In addition, the error message you
  will get from passing unix.S_ISUID and unix.S_ISGID will be different as
  they are treated as invalid bits now (note that previously passing said bits
  was also an error).

Thanks to the following contributors for helping make this release
possible:

• Aleksa Sarai cyphar@cyphar.com
• Erik Sjölund erik.sjolund@gmail.com

Signed-off-by: Aleksa Sarai cyphar@cyphar.com