copy: Introduce COPY_NOCOW_AFTER and use it when copying images

copy: Introduce COPY_NOCOW_AFTER and use it when copying images

When dealing with copying COW images, we have to make a tradeoff:

- Either we don't touch the NOCOW bit on the copied file COW and get
  an instant copy because we're able to reflink, but we might get
  reduced performance if the source file was COW as COW files and lots
  of random writes don't play well together.
- Or we force NOCOW for the copied file, which means we have to do a
  full copy as reflinking from COW files to NOCOW files or vice versa
  is not supported.

In exec-invoke.c, we've opted for the first option. In nspawn.c and
discover-image.c, we've opted for the second option.

In nspawn, this applies to the --ephemeral option to make ephemeral
copies. In discover-image.c, this applies to cloning images into
/var/lib/machines. Both these features might be used to run many
machines of the same original image. We really don't want to force
a full copy onto users in these scenarios when they're expecting
reflink behavior, leading to them running out of disk space. Instead,
degraded performance in their machines is a much less severe issue,
which they will discover on their own if it affects them, at which
point they can make their original image NOCOW at which point they'll
get both the reflinks and better performance.

Given the above reasoning, let's switch nspawn.c and discover-image.c
to use COPY_NOCOW_AFTER as well instead of enabling NOCOW upfront and
forcing a copy if the original source image is COW.