Ordering string list items on reports is also new compared to
previous state where items were not ordered at all and they
got reported simply as they appeared/were processed.
Add dm_report_reserved_value to libdevmapper for reserved value definition.
Also display field types when listing all fields in selection help.
Recognize "help" keyword in selection string to show brief help for selection.
+ Always order items reported as string list field lexicographically.
Add dm_report_field_string_list to libdevmapper for direct string list report.
Add DM_REPORT_FIELD_TYPE_STRING_LIST: separate string and string list fields.
Add dm_str_list to libdevmapper for string list type definition and its reuse.