Some additions and fixes related to fdatasync(2) and posix_fallocate(2).

* Fix a small issue in VOP_FDATASYNC()'s definition in <sys/vfsops.h>.

* Mention fdatasync() in fsync.2 (this basically sync

Some additions and fixes related to fdatasync(2) and posix_fallocate(2).

* Fix a small issue in VOP_FDATASYNC()'s definition in <sys/vfsops.h>.

* Mention fdatasync() in fsync.2 (this basically syncs that manual page
with FreeBSD, with a few adjustments).

* Add a manual page for posix_fallocate() (from FreeBSD, with a few
adjustments).

* Mention fdatasync() in the list of reentrant functions in sigaction.2.

* In pthread_testcancel.3, mention fdatasync() in the list of functions
for which a cancellation point will occur.

* Mention VOP_FDATASYNC() in VOP_FSYNC.9 (partly taken from FreeBSD).

* Remove a compatibility define in the fsstress tool.

Thanks to tkusumi for reviewing.

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