This attempts to fix <https://bugs.gnu.org/61386>, a bug in GNU cp
caused by a serious data corruption bug in FreeBSD and macOS.
* doc/posix-functions/lseek.texi: Mention the bug.
* lib/unistd.in.h (SEEK_DATA, SEEK_HOLE): Undef in macOS < 13 and
FreeBSD < 14. FreeBSD fixed the bug sometime during FreeBSD 13
<https://bugs.freebsd.org/256205>, so the "FreeBSD < 14" is
conservative. It’s unknown when Apple will fix macOS so use
macOS "9999" as a placeholder.
* m4/lseek.m4 (gl_FUNC_LSEEK): Replace lseek if on one of the
above platforms.