libdm: add dm_report_column_headings

libdm: add dm_report_column_headings

Add a function to print column headings regardless of whether they
have already been output. This will be used by dmstats to issue
periodic reminders of the column headings.

This patch removes a check for RH_HEADINGS_PRINTED from
_report_headings that prevents headings being displayed if the flag
is already set; this check is redundant since the only existing
caller (_output_as_columns()) already tests the flag before
calling the function.