New API: cpio-out converts a directory to cpio format.

This is useful for generating Linux initramfses from other types of
filesystems.  For example:

 guestfish --ro -a disk.img -i cpio-out / - | gzip -9 > initrd.img
inspect: Add support for Oracle Linux

Prior to this patch the library would return "rhel" instead of
"oraclelinux" for Oracle Linux OSes.