copydir: new package

We want a recursive copy function so that we can avoid executing /bin/cp
in supposedly isolated test suites. cp isn't always in /bin; not every /bin/cp
works the same.
I think the comments could use some explanation as to why this function exists rather than using the OS's file copy... and the difference in functionality between this and the OS file copy (e.g. change in owner, last modified date, etc).