HOWTO: Add platforms without fdatasync(2)

HOWTO: Add platforms without fdatasync(2)

Added DragonFlyBSD which is a fork of FreeBSD.
FreeBSD seems to have fdatasync(2) in their upstream, but not yet
in the latest release version if I checked correctly.

Btw, having "Windows" here sounds a bit ambiguous.
Cygwin with gcc (not MinGW) compiles below taken from ./configure
by linking against POSIX compat dll.

--
 $ uname
 CYGWIN_NT-10.0
 $ cat ./fdatasync1.c
 #include <stdio.h>
 #include <unistd.h>
 int main(int argc, char **argv)
 {
 	  return fdatasync(0);
 }
 $ gcc -Wall -g ./fdatasync1.c ; echo $?
 0

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>