cp: avoid speculative preallocation with --sparse=always

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.