copy: detect smaller holes than the copy buffer size

Previously cp would not detect runs of NULs that were
smaller than the buffer size used for I/O (currently 128KiB).

* src/copy.c (copy_reg): Use an independent hole_size, set to
st_blksize, to increase the chances of detecting a representable hole,
in a run of NULs read from the input.
(create_hole): A new function refactored from sparse_copy() and
extent_copy() so we have a single place to handle holes.
(sparse_copy): Adjust to loop over the larger input buffer
in chunks of the passed hole size.  Also adjust to only call
lseek once per hole, rather than at least once per input buffer.
* tests/cp/sparse.sh: Add test cases for various sparse chunk sizes.
* NEWS: Mention the improvement.
cp: avoid speculative preallocation with --sparse=always

With --sparse=always use fallocate(...PUNCH_HOLE...) to
avoid any permanent allocation due to speculative
preallocation employed by file systems such as XFS.

* m4/jm-macros.m4: Check for <linux/falloc.h> and fallocate().
* src/copy.c (punch_hole): A new function to try and punch
a hole at the specified offset if supported.
(create_hole): Call punch_hole() after requesting a hole.
(extent_copy): Likewise.
* NEWS: Mention the improvement.
cp: read sparse files more efficiently with non regular destination

* src.copy.c (copy_reg): Use fiemap to read sparse files, even
if the output is not to a regular file.
* NEWS: Mention the improvement.
copy: avoid an extraneous error when reporting errors

* src/copy.c (copy_reg): If sparse_copy() failed, then an
erroneous error about failing to extend the file would be reported.
chroot: call chroot() unconditionally to handle bind mounted "/"

* src/chroot.c (is_root): Adjust to compare canonicalized paths
rather than inodes, to handle (return false in) the case where
we have a tree that is constructed by first bind mounting "/"
(thus having the same inode).
(main): Unconditionally call chroot() because it's safer
and of minimal performance benefit to avoid in this case.
This will cause inconsistency with some platforms
not allowing `chroot / true` for non root users.
* tests/misc/chroot-fail.sh: Adjust appropriately.
* NEWS: Mention the bug fixes.
Fixes http://bugs.gnu.org/18736
tests: fix test hang with unstable inodes in /proc

* cp/proc-zero-len.sh: Search the 'err' file
for the error to ignore, not stdin.