cp: copy sparse files efficiently using the FIEMAP ioctl

cp: copy sparse files efficiently using the FIEMAP ioctl

* src/fiemap.h: Add fiemap.h for fiemap ioctl(2) support.  Copied
from linux's include/linux/fiemap.h, with minor formatting changes.
* src/copy.c (copy_reg): Now, when `cp' is invoked with --sparse=[WHEN],
we will try to do FIEMAP-copy if the underlying file system
supports it, and fall back to a normal copy if it fails.