config: thin_repair_options and cache_repair_options are undefined

config: thin_repair_options and cache_repair_options are undefined

By default these are empty strings, so the config settings
should be flagged as undefined, so they will be commented
out of the generated config.  Otherwise, the lines:

thin_repair_options=""
cache_repair_options=""

in the dump output cause a warning when processed since
lvm doesn't want an empty string.

Also regenerate lvm.conf.in.