mtr-packet: drop capabilities + using BSD's linked lists for probes

mtr-packet: drop capabilities + using BSD's linked lists for probes

At startup, we now use cap_set_proc to drop all privileged
capabilities for the mtr-packet process.  This means that
capabilities granted through the commandline setcap to the
mtr-packet executable will only be in effect while the necessary
raw sockets are opened, and will be dropped before any command
requests are read.

Now we use BSD's queue.h linked list support for storing outstanding
probes.  This makes iterating through in-flight probes more efficient,
as we don't need to loop through many unused probe entires when only
a few probes are outstanding.

Changed mtr-packet's default probe size to 64 bytes, to match
mainline mtr's default.

The code consistently uses 'exit(EXIT_FAILURE)' instead of 'exit(1)'.
The effect is the same, but the intent is clearer.