Initial ICMP support

Created by: batonius

Problem: Redox doesn't support ICMP, it doesn't respond to ECHO requests, it doesn't have the ping utility.

Solution: Introduce the icmpd daemon to handle incoming ICMP messages and provide file system abstraction of ICMP to other processes via the icmp: scheme.

Changes introduced by this pull request:

• The icmpd daemon has been added.
• The daemon replies to incoming ICMP ECHO_REQUEST packets with valid ECHO_RESPOND packets.
• The daemon listens to ip:1 and creates the :icmp scheme.
• The new scheme supports echo/<ip-addr> paths to interact with ICMP.
• Writing to an echo/* file sends an ICMP ECHO_REQUEST packet with the buffer argument as the packet's payload. icmpd sets the packet's id field to the value of the file handle used.
• An echo/* file produces an event on incoming ICMP ECHO_RESPONSE with id equal to the id assigned to the file handle.
• Reading from an echo/* file produces payload from the received packet.

Drawbacks:

• Client processes has no access to the underlying network headers. For example, they can't check TTL of a response.

State: Redox replies to incoming ECHO requests and is able to send ECHO requests and process ECHO responses via the ping utility.

TODOs:

• Add support for *_UNREACHABLE messages to icmpd.
• Add *_unreachable methods to icmpd scheme.
• Add support for icmpd to ipd, udpd and tcpd.

Other: The qemu's default netwok settings don't support forwarding of ICMP packets, so additional configuration is required. Here's the configuration I'm using.