activation: flag temporary LVs internally

Add LV_TEMPORARY flag for LVs with limited existence during command
execution. Such LVs are temporary in way that they need to be activated,
some action done and then removed immediately. Such LVs are just like
any normal LV - the only difference is that they are removed during
LVM command execution. This is also the case for LVs representing
future pool metadata spare LVs which we need to initialize by using
the usual LV before they are declared as pool metadata spare.

We can optimize some other parts like udev to do a better job if
it knows that the LV is temporary and any processing on it is just
useless.

This flag is orthogonal to LV_NOSCAN flag introduced recently
as LV_NOSCAN flag is primarily used to mark an LV for the scanning
to be avoided before the zeroing of the device happens. The LV_TEMPORARY
flag makes a difference between a full-fledged LV visible in the system
and the LV just used as a temporary overlay for some action that needs to
be done on underlying PVs.

For example: lvcreate --thinpool POOL --zero n -L 1G vg

- first, the usual LV is created to do a clean up for pool metadata
  spare. The LV is activated, zeroed, deactivated.

- between "activated" and "zeroed" stage, the LV_NOSCAN flag is used
  to avoid any scanning in udev

- betwen "zeroed" and "deactivated" stage, we need to avoid the WATCH
  udev rule, but since the LV is just a usual LV, we can't make a
  difference. The LV_TEMPORARY internal LV flag helps here. If we
  create the LV with this flag, the DM_UDEV_DISABLE_DISK_RULES
  and DM_UDEV_DISABLE_OTHER_RULES flag are set (just like as it is
  with "invisible" and non-top-level LVs) - udev is directed to
  skip WATCH rule use.

- if the LV_TEMPORARY flag was not used, there would normally be
  a WATCH event generated once the LV is closed after "zeroed"
  stage. This will make problems with immediated deactivation that
  follows.