This first commit showcases some basic functionality. It currently only
works in text mode and with color disabled. For this to work:
 - I added an agflag (*a*bsolute *g*raph) which is for now always set to
   1 (enabled)
 - I changed the signature of the print_bar method and their
   implementations to consume three doubles:
   the used & total space on this volume as well as the total space on
   the greatest volume
 - I added an maxfssize variable alongside the max variable that stores
   the greatest volume's total space which is calculated whenever the
   latter is updated (see util.c)
Every other impl other than the one in text is simply a stub for now.

I decided to go with -A, it is off by default (default behaviour
is the same as before)
I also added documentation in the english man page and help text,
translated versions still need to add documentation for this as I
speak neither french nor dutch :)

in absolute graph mode, it is wider than the other bars, so I needed
support for flexible graph / bar width. before this commit, the color
steps in the absolute summary graph would be on the same level as the
ones above but not at the correct percentages, think something like this:
sda1 ggggggyyyyrr--
sda2 ggggggy-------
sda3 ggggggyy------
sum: ggggggyyyyrrrrrrrrrrrrrrrrr---------------
but it should look like this:
     ggggggggggggggggggyyyyyyyrr---------------
This issue is fixed in this commit.

If dfc decides at runtime to only display short  bars instead of wide
bars, because the tty is too narrow, the `max.bar` still indicates
`GRAPHBAR_WIDE` but it should be `GRAPHBAR_SHORT`.
So for calculating the detail fields offset I am now using the ternary
thing instead of `max.bar`.

This requires another look into it because currently, the titlebar
is off if we are using the `-w` option and the tty is too narrow.