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)

Avoid compiler errors from new versions

rename flag

add documentation and spaces
Merge pull request #2784 from kuettler/issue_11434

Issue 11434: std.file.copy doesn't preserve file attributes (eg: executable mode etc)