Implement support for flush, fua and rotational.

Implement support for flush, fua and rotational.

This commit implements support for the flush, fua, and rotational directives
within the configuration file.

FUA means "force the current write to hit the media service" and FLUSH
means "empty the current write queue to disk". Broadly they have the same
semantics as the linux kernel REQ_FLUSH and REQ_FUA. FUA is implemented
through sync_file_range() (or if that doesn't exist, fdatasync() on the
file handle concerned), FLUSH through fsync() on all files. FUA and
FLUSH are selected in the config file, and set new flags bits which will
cause the client to sent FUA and FLUSH requests. The way these are
implemented is further explained in doc/proto.txt.

The purpose of this is reasonably obvious: without supporting either FUA
or FLUSH (and it's relatively easy to support both), filesystems on the
client have no way to ensure the relevant sectors have hit the disk.
The patch is implement such that the default behaviour is unchanged.

Additionally, it introduces an F_ROTATIONAL flag. This will turn off
the use of QUEUE_FLAG_NONROT in the client. QUEUE_FLAG_NONROT effectively
disables the elevator algorithm, making the algorithm merge only. That is
unhelpful where the server does not have its own elevator algorithm
or where the client elevator algorithm is neutered (e.g. writing to a
raw partition with F_SYNC with nbd-server). It's not going to be used
often where the backing store is a file.

It also incidentally fixes a bug where F_SYNC is ignored if F_COPYONWRITE
is set (not that this currently has much utility).

Note the following:

* The top 16 bits of the command type have been reserved
  (see NBD_CMD_MASK_COMMAND) for passing flags to be attached to commands.
  NBD_CMD_FLAG_FUA is the first of these.

* simple_test has been modified so it does not stomp over nbd.conf and
  pid files in the current directory.

* A new ioctl has been added which passes the export flags to the kernel.
  There is currently no support for this in the kernel (I will submit
  a patch in due course)

* "make check" now performs a flush and fua test. These have been verified
  by strace to "do the right thing".

* nbd-client incorrectly shifted the flags value left by 16 after reading it.
  This caused the test of the readonly bit to always fail. I suspect this
  may have been an attempt to combine server flags and export flags into
  a single 32 bit word. However, the low 16 bits were never set, and only
  the low 16 bits are tested. As the only thing it was testing for was the
  read only flag, and that is supplied in the flags, and as this allows
  transparent passing to the ioctl, I suggest it is changed as per this commit.
  If there's something else to add, it can be put in the upper 16 bits.

I have tested this lightly with nbd-client (obviously the kernel does not
yet send FLUSH or FUA) and with the test suite. It's probably ready to
go in marked "experimental".