lvmconfig: do not display settings with undefined default values

lvmconfig: do not display settings with undefined default values

Do not display settings with undefined default values, but do display
these settings in case the value is defined directly in any part of
the existing config cascade.

For example, the lvmconfig --type current always displays these settings
(as it's somewhere in "current" configuration cascade that makes it defined).
The lvmconfig --type full displays these settings only if it's defined
somewhere in the cascade, but not if default value is used instead
The lvmconfig --type default never displays these settings...

More concrete example - let's have activation/volume_list directly
set in lvm.conf and activation/read_only_volume_list not set.
Both of these settings have *undefined default* values.

  $lvmconfig --type full activation/volume_list activation/read_only_volume_list
  volume_list="/dev/vg/lv"

(...only volume_list is defined, hence it's printed)

However, the comments will display more info (see also previous commit):

  $lvmconfig --type full activation/volume_list activation/read_only_volume_list --withsummary

  # Configuration option activation/volume_list.
  # Only LVs selected by this list are activated.
  # This configuration option does not have a default value defined.
  # Value defined in existing configuration has been used for this setting.
  volume_list="/dev/vg/lv"

  # Configuration option activation/read_only_volume_list.
  # LVs in this list are activated in read-only mode.
  # This configuration option does not have a default value defined.