report: support printing full column names in report headings

report: support printing full column names in report headings

Reuse existing report/headings config setting to make it possible to
change the type of headings to display:
  0 - no headings
  1 - column name abbreviations (default and original functionality)
  2 - full column names (column names are equal to exact names that
      -o|--options also accepts to set report output)

Also, add '--headings none|abbrev|full|0|1|2' command line option
so we are able to select the heading type for each LVM reporting
command directly.