non-zero process exit status when too many packets are unreturned

The number of unreturned packets in excess of 50% is summed over all
hosts.  If this is nonzero, this count plus 1 is the process exit
status.
oping: Implement the "-Z" option.

This makes it possible to configure the threshold for exiting with a non-zero
exit status due to missing replies.
fiddle with failure threshold messages and comments
oping: Make the exit status the number of hosts failed.

As far as I know, EXIT_FAILURE is only portable when used as-is, i.e. without
adding anything to it. Especially with "EXIT_FAILURE == -1" we might run into
trouble. I think it's more convenient to just use the number of failed hosts
rather than 1+<num> (on Linux).
Merge branch 'bp/exit'

Conflicts:
	src/mans/oping.pod
	src/oping.c