Wait for udev change on /dev/DISK when querying whole device FS (!46)

GParted nearly always shows this warning against a whole disk device
FAT32 file system on my development VMs:
    plain floppy: device "/dev/sdb" busy (Resource temporarily
    unavailable): Cannot initialize '::'
    mlabel: Cannot initialize drive
However GParted does get the label via blkid instead.  Occasionally
everything works correctly and there is no warning.

Never found a similar error for any other file system on a whole disk
device.  The timing never seems to clash.

Remember that when a writable file descriptor of a disk device is
closed, udev events are triggered which update the kernel and /dev
entries for any partition changes.  (This is in addition to coding which
has always existed in the partitioning tools, including fdisk and
parted, to inform the kernel, but not create /dev entries, of partition
changes).

With debugging and sleeping in GParted, stracing the GParted thread
GParted_Core::set_devices_thread() and using 'udevadm monitor' to watch
udev events the following sequence of events is observed:

  gparted    |set_devices_thread(pdevices)  pdevices->["/dev/sdb"]
  gparted    |  ped_device_get("/dev/sdb")
  libparted  |      open("/dev/sdb", O_RDONLY) = 8
  libparted  |      close(8)
  gparted    |  useable_device(lp_device)  lp_device->path="/dev/sdb"
  gparted    |    ped_device_open()
  libparted  |      open("/dev/sdb", O_RDWR) = 8
  gparted    |    ped_device_read()
  gparted    |    ped_device_close()
  libparted  |      close(8)
  udev(async)|        KERNEL change /devices/.../sdb (block)
  udev(async)|        KERNEL change /devices/.../sdb (block)
  udev(async)|        UDEV   change /devices/.../sdb (block)
  udev(async)|        UDEV   change /devices/.../sdb (block)
  gparted    |  set_device_from_disk(device, device_path="/dev/sdb")
  gparted    |    get_device(device_path="/dev/sdb", ..., flush=true)
  gparted    |      ped_device_get()
  gparted    |      flush_device(lp_device)  lp_device->path="/dev/sdb"
  gparted    |        ped_device_open()
  libparted  |          open("/dev/sdb", O_RDWR) = 8
  gparted    |        ped_device_sync()
  gparted    |        ped_device_close()
  libparted  |          close(8)
  udev(async)|            KERNEL change /devices/.../sdb (block)
  udev(async)|            KERNEL change /devices/.../sdb (block)
  udev(async)|            UDEV   change /devices/.../sdb (block)
  udev(async)|            UDEV   change /devices/.../sdb (block)
  gparted    |    set_device_one_partition()
  gparted    |      set_partition_label_and_uuid()
  gparted    |        read_label()
  gparted    |          fat16::read_label()
  gparted    |            Utils::execute_command("mlabel -s :: -i /dev/sdb")

This sequence of events only shows which close()s by libparted trigger
udev events.  It does not show that processing those events are
asynchronous and overlap with GParted executing mlabel, causing the
device busy error.

As libparted's ped_device_open() doesn't offer a way to specify that a
device will only be used for reading, it always opens it read-write
[1][2].  Hence this sequence in GParted_Core::flush_device():
    ped_device_open()
    ped_device_sync()
    ped_device_close()
always triggers udev change events on the whole disk device.  Fix by
waiting for those udev events to complete.

[1] libparted documentation, PedDevice, ped_device_open()
    https://www.gnu.org/software/parted/api/group__PedDevice.html#ga7c87bd06e76553c8ff3262b5e6ca256

[2] parted/libparted/arch/linux.c:linux_open()
    http://git.savannah.gnu.org/cgit/parted.git/tree/libparted/arch/linux.c?id=v3.2#n1628

Closes !46 - Whole device FAT32 file system reports device busy warning
             from mlabel

src/GParted_Core.cc

@@ -4090,6 +4090,12 @@ bool GParted_Core::flush_device( PedDevice * lp_device )
 	{
 		success = ped_device_sync( lp_device ) ;
 		ped_device_close( lp_device ) ;
+
+		// (!46) Wait for udev rules to complete after this ped_device_open() and
+		// ped_device_close() pair to avoid busy /dev/DISK entry when running file
+		// system specific querying commands on the whole disk device in the call
+		// sequence after get_device() in set_device_from_disk().
+		settle_device(SETTLE_DEVICE_PROBE_MAX_WAIT_SECONDS);
 	}
 	return success ;
 }