Proper way to do setcap in maintscript

[Niels Thykier <niels@thykier.net>]

Hi,

I have seen the following pattern in multiple packages, where we use 
`setcap` to replace a setuid (or setgid) mode with a capability. I think 
it is about time that we get proper packaging helper support for it.


```
if [ "$1" = configure ]; then
     if command -v setcap > /dev/null; then
         if setcap CAP CMD then
             chmod u-s CMD
         else
             echo "Setcap failed ..." >&2
         fi
     else
         echo "Setcap is not installed,  ..." >&2
     fi
fi
```

If I was to add support for this snippet in package helpers, is there 
anything I should change in it? Such as:

 * Should the snippet use dpkg-statoverride instead of a chmod?
   (If dpkg-statoverride is used, how will this interact with the next
    bullet?)
 * Should the snippet use $DPKG_ROOT for the CMD even though setcap
   would presumably have to be run from the HOST system?

The snippet format has been used for a while, so it definitely "works". 
But I figured the basic template could do with a review to see if it is 
still up to speed with best practices - especially if we start adding it 
to a package helper. :)

Best regards,
Niels

PS: I am also happy to receive suggestions for how to integrate this 
better with dpkg. My understanding though is that it will come with the 
dpkg manifest format, so I assumed the package helper just had to do 
some maintscript glue for now.