Add ping utility

Created by: batonius

Problem: Redox doesn't have the ping utility.

Solution: Introduce the ping utility with minimal required features.

Changes introduced by this pull request:

• The ping utility has been added.
• The utility close follows the behavior of the ping utility from iputils.
• The utility supports both IPv4 and host name destinations.
• The utility supports -h, -c and -i options.
• The utility doesn't require root privileges, suid bit or raw_net capability.
• Several instances of the utility are able to work in parallel without interfering.

Drawbacks:

• The utility has no access to the TTL field of the response packet.
• Without the -c argument ping sends requests indefinitely and can't be interrupted with Ctrl-C. It provides no signal handlers and should be terminated on SIGINT, I suppose it has something to do with the current implementations of signals in Redox.

State: Redox has ping utility with the traditional behavior.

Other: This PR requires redox-os/netstack#1.