Add more options than "if_newer" to fs.copy

[atollk]

fs.copy offers free functions to copy a file system / directory / single file only if the source is newer than the destination. Those functions are marked by the _if_newer suffix.

I propose to add instead variants with just an _if suffix that take a strategy: str parameter to distinguish between different conditional cases rather than having to add three new _if_X functions. In particular, it would make supporting the following three strategies in addition to "if_newer" easy:

• Only copy if the source is older than the destination.
• Only copy if the destination does not exist.
• Only copy if the destination already exists.