Windows does not provide cat.  Instead, copy the test source to stdout
using the file stream to which it was written.

Changes since v1:
- Create fwrite_noeintr to avoid EINTR in fwrite without writing any
  data.
- Handle short reads from fread.  This can happen with non-conformant
  libc or if EINTR occurs after reading some data.
- Handle short writes from fwrite.  This can happen with non-conformant
  libc or if EINTR occurs after writing some data.

Changes since v2:
- Revert fwrite_noeintr and short read/write changes.
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>