user: fix  function signatures

As @crosbymichael pointed out [in this discussion](https://github.com/docker/docker/pull/9046#discussion_r20268452), the functions in the `user` package (specifically the lookup API) are not correctly named. This patchset fixes this.

Specifically, the following function names were changed to make the API more intuitive:
* `GetGroupFile` => `GetGroupPath`
* `GetPasswdFile` => `GetPasswdPath`
* `GetExecUserFile` => `GetExecUserPath`
* and several unexposed internal functions were renamed in a similar fashion.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)