thin: new pool is activated without overlay

thin: new pool is activated without overlay

Activate of new/unused/empty thin pool volume skips
the 'overlay' part and directly provides 'visible' thin-pool LV to the user.

Such thin pool still gets 'private' -tpool UUID suffix for easier
udev detection of protected lvm2 devices, and also gets udev flags to
avoid any scan.

Such pool device is 'public' LV with regular /dev/vgname/poolname link,
but it's still 'udev' hidden device for any other use.

To display proper active state we need to do few explicit tests
for this condition.

Before it's used for any lvm2 thin volume, deactivation is
now needed to avoid any 'race' with external usage.