Unix: add ping6 (closes #821)

I simply copy-pasted everything and changed `ping` to `ping6` and replaced `127.0.0.1` with `::1`.

The separate commits are in my `unix` branch in case they are needed before this is merged.

Tests passed on localhost and I tested that this worked with Tessu.

```
2014-08-24 17:42:21+0300 <@Mikaela> %reload Unix
2014-08-24 17:42:22+0300 -- Notice(Tessu): OK!
2014-08-24 17:42:26+0300 <@Mikaela> %list unix
2014-08-24 17:42:29+0300 -- Notice(Tessu): call, crypt, errno, fortune, pid, ping, ping6, progstats, shell, spell, sysuname, sysuptime, ja wtf
2014-08-24 17:42:34+0300 <@Mikaela> %help ping6
2014-08-24 17:42:37+0300 -- Notice(Tessu): (ping6 [--c <count>] [--i <interval>] [--t <ttl>] [--W <timeout>] <host or ip>) -- Sends an ICMP echo request to the specified host. The arguments correspond with those listed in ping6(8). --c is limited to 10 packets or less (default is 5). --i is limited to 5 or less. --W is limited to 10 or less. 
2014-08-24 17:42:51+0300 <@Mikaela> %unix ping6 ::1
2014-08-24 17:42:56+0300 -- Notice(Tessu): : 5 packets transmitted, 5 received, 0% packet loss, time 4013ms rtt min/avg/max/mdev = 0.045/0.050/0.056/0.009 ms 
2014-08-24 17:43:03+0300 <@Mikaela> %unix ping6 ipv6.google.com
2014-08-24 17:43:10+0300 -- Notice(Tessu): arn02s06-in-x04.1e100.net: 5 packets transmitted, 5 received, 0% packet loss, time 4071ms rtt min/avg/max/mdev = 64.083/64.142/64.186/0.036 ms               
```