vnstat: Update to 2.7

vnstat: Update to 2.7

For details see:

https://humdi[dot]net/vnstat/CHANGES

"2.7 / 16-May-2021

 - Fixed
   - Possibility of segmentation fault with image list output when database
     existed but no data was available
   - ./configure output could show invalid install paths with some parameter
     combinations (pull request by Severin Glöckner)
   - Columns in text hours graph output could get misaligned if the selected
     system locale used a UTF-8 sequence for the thousands separator instead
     of a single character
 - New
   - Add -5g / --fivegraph options to image output with sizing related
     parameters for the output of a 5 minute resolution bar graph
   - Add configuration option SummaryGraph and optional parameter for
     --hsummary and --vsummary for selecting which graph is shown next to
     the summary data in the horizontal and vertical summary image outputs
   - Add --large / --small options and configuration option LargeFonts for
     controlling the image output font size
   - Add --scale and configuration option ImageScale for scaling the image
     output to a given percent
   - Add configuration option LineSpacingAdjustment for adjusting the line
     spacing of list format image outputs
   - Add bar visualizations for traffic estimations in image output
   - Allow writing image output to a filename starting with -
   - Add --initdb to daemon for creating a new empty database without having
     the daemon process staying running, doesn't discard data if a database
     already exists
   - Add configuration option BarColumnShowsRate for having the bar column in
     image list outputs be scaled according to the average rate column values
     when those values are visible, disabled by default
   - Add --dbiflist for getting a list of interfaces in the database, both
     --iflist and --dbiflist also get alternative more parseable outputs
   - Add configuration option for large font output and make 5 minute
     resolution graph visible in vnstat.cgi"

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>