From Petr Lautrbach: when capturing, return the exit status of dumpcap,

From Petr Lautrbach: when capturing, return the exit status of dumpcap,
so we give a non-zero exit status for invalid interfaces or capture
filters.

From me: don't exit immediately if dumpcap failed, print out information
from taps and the like.

svn path=/trunk/; revision=33393