fdatasync: minor improvements Picks up some ideas by Paul Eggert from here: https://lists.gnu.org/archive/html/bug-coreutils/2011-07/msg00157.html * modules/fdatasync (Depends-on): Add condition for fsync. * lib/fdatasync.c (fdatasync): Add comment. * tests/test-unistd-c++.cc: Test fdatasync. Signed-off-by: Eric Blake <eblake@redhat.com>