Implementing the logic that is in-flight with [1], but using recursive
removal [2].  GNU tar has a --recursive-unlink option that's not
enabled by default, with the motivation being something like "folks
would be mad if we blew away a full tree and replaced it with a broken
symlink" [3].  That makes sense for working filesystems, but we're
building the rootfs from scratch here so losing information is not a
concern.  This commit always uses recursive removal to get that old
thing off the filesystem (whatever it takes ;).

The exception to the removal is if both the tar entry and existing
path occupant are directories.  In this case we want to use GNU tar's
default --overwrite-dir behavior, but unpackLayer's metadata handling
is currently very weak so I've left it at "don't delete the old
directory".

The reworked directory case also fixes a minor bug from 44210d0
(cmd/oci-image-tool: fix unpacking..., 2016-07-22, opencontainers#177) where the:

  if fi, err := os.Lstat(path); !(err == nil && fi.IsDir()) {

block would not error out if the Lstat failed for a reason besides the
acceptable IsNotExist.  Instead, it would attempt to call MkdirAll,
which would probably fail for the same reason that Lstat failed
(e.g. ENOTDIR).  But it's better to handle the Lstat errors directly.

[1]: opencontainers/image-spec#317
[2]: https://github.com/opencontainers/image-spec/pull/317/files#r79214718
[3]: https://www.gnu.org/software/tar/manual/html_node/Dealing-with-Old-Files.html

Signed-off-by: W. Trevor King <wking@tremily.us>

To help address:

  $ make lint
  checking lint
  image/manifest.go:140::warning: cyclomatic complexity 39 of function unpackLayer() is high (> 35) (gocyclo)
  ...

Signed-off-by: W. Trevor King <wking@tremily.us>