unshare: create a new mount namespace
unshare: create a new mount namespace

it is needed to support other backends than vfs.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1124
Approved by: rhatdan

TomSweeneyRedHat/buildah,rhatdan/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,ipbabble/buildah,nalind/buildah,nalind/buildah,rhatdan/buildah