Create common POSIXArchiveEntry for use with tar/cpio/dump

There's currently a top-level ArchiveEntry but many archivers are for POSIX (or POSIX-like) filesystems with standard attributes - ownership, permissions, access times, etc. By creating a common ArchiveEntry class for these attributes it will be much easier to write archive-agnostic extraction software. (Many of the existing methods should be marked 'deprecated' but left in place indefinitely.)

I have a prototype of this class in my dump patch.