gmetad: Add dual-stack support for data_sources and riemann export.

Now data sources can be IPv4 and IPv6 addresses or hostnames like that:
```
data_source "source" 192.168.0.2 192.168.0.3:8655
data_source "another source" node1.example.com node2.example.com:8655
data_source "yet another source" fd25:d3b4:365d:2::4 [fd25:d3b4:365d:2::5]:8655
```
Riemann host also can be IPv4/IPv6/hostname:
```
riemann_server 127.0.0.1
riemann_server ::1
```