stage1: improve duplicate mount-volume detection

[lucab]

Breaking change: volumes with duplicate names are now rejected.

This PR adds error-checking for duplicate volume names. Also, it adds path cleaning when de-duplicating mounts targets. Tests are slightly adjusted to check both cases.

Fixes #3663

[lucab]

/cc @euank

See appc/spec#687 for the corresponding fix directly in appc/spec.

[euank]

This does not fix all of #3663

The important one is that multiple mounts with the same target should work if they have different names, but that case still doesn't work with this PR.

Specifically:

sudo rkt run --stage1-name=coreos.com/rkt/stage1-fly:1.25.0 --insecure-options=image,ondisk \
   --volume y,kind=host,source=/tmp \
   --mount volume=x,target=/tmp \
   --volume x,kind=host,source=/tmp \
   --mount volume=x,target=/tmp \
   docker://busybox --exec=/bin/sh -- -c 'exit 0' && echo success

run: can't evaluate mounts: missing mount for volume "y"

[lucab]

@euank This is because stage0 is de-duplicating those mounts. Other stage1s work because they don't check that all volumes are covered by a mount. Looking at pod spec I think fly is being too strict in performing this check. I'll queue a commit on top of this PR to relax it.

[lucab]

Rebased on master, PTAL.

@euank can you re-check if this now implements the proper behavior from wrapper flags point of view?

[euank]

I think this allows a solution to our wrapper woes.

The other changes LGTM as well.