This patch modifies way of creating pidfiles from /var/run/fprobe.pid to
/var/run/fprobe-<interface>.pid ( argument -i ) which allows to run 
simulatnously several instances of fprobe on single Debian box.

Example:

somewhere:/home/vnull/build/fprobe-ng-1.1# ls -1 /var/run/*fprobe*
/var/run/fprobe-eth1.10.pid
/var/run/fprobe-eth1.20.pid
/var/run/fprobe-eth1.pid