Fix snapshot segment import to not use duplicate segments & replace.

Fix snapshot segment import to not use duplicate segments & replace.

The snapshot segment (snapshotX) is created twice
during the text metadata segment processing.

This can cause temporary violation of max_lv count.

Simplify the code, snapshot segment is properly initialized
in init_snapshot_seg function now and do not need to be replaced
by vg_add_snapshot call.

The vg_add_snapshot() is now usefull only for adding new
snapshot and it shares the same initialization function.

The snapshot name is always generated, name paramater can be
removed from function call.