lvconvert: more support for yes conversion

lvconvert: more support for yes conversion

When converting volume to pool LV use also wiping of other signatures.
For writecache & pool conversion support --yet and --force
to bypass prompting for signature wiping.
For writecache drop unneded zero_sectors.

Note: currently we have lvconvert doing convertion and prompting
for confirmation of conversion - and then again wipe_lv() prompts
for removing i.e. filesystem signature - we should unify this
prompting into 1 message  - althought the 'filesystem' discovery
needs active volume - while the 1st. conversion prompt can
work without active converted volume.