Add a method to copy files, preserving attributes

Summary:
Unlike the built-in copy() function, this preserves file attributes
on filesystems which have them.

See D16273.

Test Plan: Called from arcanist's ArcanistLintPatcher

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16275