* 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' invoked with --sparse=[WHEN] option, we
will try to do FIEMAP-copy if the underlaying file system support it, fall back
to a normal copy if it fails.

* tests/cp/sparse-fiemap: Add a new test for FIEMAP-copy against a
loopbacked ext4 partition.
* tests/Makefile.am (sparse-fiemap): Reference the new test.

* tests/cp/sparse-fiemap: Cd into test directory sooner.

* tests/cp/sparse-fiemap: Ensure that a file with many extents (more
than fit in copy.c's internal 4KiB buffer) is copied properly.

* tests/cp/sparse-fiemap: Don't require root access if current
partition is btrfs or xfs.
Use init.sh, not test-lib.sh.

* tests/cp/sparse-fiemap: More tests.

* cfg.mk (sc_root_tests): Allow spaces before "require_root_",
now that tests/cp/sparse-fiemap has a conditional use.

* tests/cp/sparse-fiemap: Improve.
* tests/filefrag-extent-compare: New file.

* src/copy.c (fiemap_copy): Rename from fiemap_copy_ok.
Add/improve comments.
Remove local, "fail".
(fiemap_copy): Do not require caller to set
"normal_copy_required" before calling fiemap_copy.
Report ioctl failure if it's the 2nd or subsequent call.

* src/copy.c (fiemap_copy): Ensure that our fiemap buffer
is large enough and well-aligned.
Replace "0LL" with equivalent "0" as 3rd argument to lseek.

* src/fiemap.h: Include stdint.h, not linux/types.h,
now that this file uses only portable type names.

* tests/cp/sparse-fiemap: Accommodate values other than "eof"
in the "flags" column of filefrag -v output

* src/copy.c (fiemap_copy): Force kernel to sync the source
file before mapping.

* src/Makefile.am (noinst_HEADERS): Add fiemap.h.

* tests/Makefile.am (EXTRA_DIST): Add filefrag-extent-compare.