Move cp(1)-like permission changes from copy to cp,

Move cp(1)-like permission changes from copy to cp,

and add setid bit handling

- Moves the permission-changing code to File::Copy::cp(), making it
non-identical to File::Copy::copy().  I think this is a decent
compromise, retaining backwards compatibility in the "primary"
routine, while allowing people who chose cp() to get more POSIXy
semantics.  It does break backwards compatibility of cp() wrt
permissions.  I can find a handful of examples in CPAN, for which I
hope smokers will give some feedback.  As for the DARKpan, who knows .
. .

- Expands the permission changing in cp() a bit to cover the rest of
what cp(1) claims on the local POSIX box.

- Removes Fcntl from the mix, to eliminate some build-time hoops.

- Shifts Abigail's permission tests to cp(), and adds parallel tests for copy().

- Documents the permission-related behavior in more detail.