patch: teaching dump(8) to change tapes

Hello,

Wrote a small and simple patch to teach dump(8) to change tapes via a 
external script.

This sure helps me, not sure if it's ready for or even wanted for 
commit though :)

I think using a script for this is wise because there is a myriad of 
tape robot systems out there, pair that with a myriad of different 
sysadmins with a myriad different backup strategies, and you got a 
situation impossible to map in a generic program such as dump, let them 
sh or perl it themselves.

Any comments?

Adds "-F /some/where/script" as an option.

The script gets device name and current volume number passed on the 
command line.

Name of the option and arguments to the script are inspired by the GNU 
counterpart (as of the man page).  If we didn't think of it first, 
let's at least make it compatible :)

mvh,
Frode