Don't import std.format in any public scope.

Workaround for: https://issues.dlang.org/show_bug.cgi?id=13808
Merge pull request #2780 from quickfur/issue13808

Issue 13808: Don't import std.format in any public scope.
fix issue 11434
added version(Posix) to unittest
Add PreserveAttributes flag to copy function to preserve file attributes (issue 11434) and current behavior (not at the same time)