lvconvert: add missing option for repair

Few more option needs to be allowed with --repair.
(in-release fix).
lvconvert: move to single name validation

Validate all LV names in _lvconvert_name_params().
lvconvert: improve splitsnapshot test

Easier check for conflicting options with --splitsnapshot.
lvconvert: improve merge validation

Easier check for conflicting options with --merge.
cache: use get_cache_mode for validation

Use a single function to validate cache mode arg
and set DM_ feature flags.
metadata: use outfc for comments

Few unecessary comments were written to on-disc metadata.
Use outfc() to have comments only in archived files.
(may also save couple bytes in ringbuffer).

TODO: needed validation against newline char...
raid: support lvdisplay --maps

Add legs printing for --maps
Somewhat similar to mirror support - maybe there are more things to
show...
cleanup: check arg_count once

Do not check quiet_ARG more then necessary.
cleanup: typo in message

in-release fix.
cleanup: drop testing impossible path

We cannot get NULL in this test - since if the arg is set
it will always return non-NULL value here.
(in-release update)
tests: use bigger metadata

Until resolved - use bigger then 4MB cache pool metadata.