Fix syntax errors, ICMP logic errors. Add ping.

Fixes:
 - ICMP_TEMPLATE decode had missing quotes and a semicolon.
 - arp_request and arp_reply send the correct protocol values.
 - ipv4_process_queue operates in place.
 - Properly refactor names in icmp_event after event changes.
 - Order arguments to ipv4_send in client.lua correctly.
 - Have icmp_event reply to the sender, not to itself.

Changes:
 - Increase default IPv4 TTL to 128 to fit commonly seen practical
   values. (Based on Windows. 64 supposedly is the standard, though?)
 - Add filtering support for ICMP. May still need to handle promiscuity.
 - Add own address to the arp_cache on initialization.
 - Cache hardware addresses for received packets.
 - Change ping payload to os.clock for easier subtraction at the expense
   of not allowing other hosts to get origin timestamps.

Additionally, add a 'ping' application for pinging remote hosts.