Bug: df does not have a --total option

[Dan Reish]

Everyone needs a --total option.

The patch I've attached has undetected overflow issues.  I'm sure it
needs to be cleaned up a little, but I'm not sure how to do it.  Maybe
by detecting whether the total just summed is less than the thing
added to it.  It works on my system with my sub-terabyte levels of
disk storage, though.

Also, I used -o as the short form.  I'm not sure whether POSIX has a
problem with that.

df-patch
Description: Text document