Move the identical static `chmod_` functions in libstore to
libutil. the function is called `chmodPath` instead of `chmod`
as otherwise it will shadow the standard library chmod in the nix
namespace, which is somewhat confusing.
Change-Id: I7b5ce379c6c602e3d3a1bbc49dbb70b1ae8f7bad
Artemis Tosini
5411fbf204