ip_address: make external IP address source configurable

[pdecat]

The following URLs used to report the external IP address are currently inaccessible with my K-Net ISP:

- http://v4.ipv6-test.com/api/myip.php
- http://v6.ipv6-test.com/api/myip.php

When I try to access them from my main ISP, I get a timeout:

```
# wget -4 -O- --timeout=5 --tries=1 --progress=dot http://v4.ipv6-test.com/api/myip.php
--2020-03-28 11:36:32-- http://v4.ipv6-test.com/api/myip.php
Resolving v4.ipv6-test.com (v4.ipv6-test.com)... 51.75.78.103
Connecting to v4.ipv6-test.com (v4.ipv6-test.com)|51.75.78.103|:80... failed: Connection timed out.
Giving up.
```

Somehow, either there's a routing issue or my IP address is blocked.

When using my backup Free ISP or a VM on Google Cloud, no problem:

```
# wget -4 -O- --timeout=5 --tries=1 --progress=dot http://v4.ipv6-test.com/api/myip.php
--2020-03-28 11:35:48-- http://v4.ipv6-test.com/api/myip.php
Resolving v4.ipv6-test.com (v4.ipv6-test.com)... 51.75.78.103
Connecting to v4.ipv6-test.com (v4.ipv6-test.com)|51.75.78.103|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: ‘STDOUT’
82.64.xxx.xxx
     0K 749K=0s

2020-03-28 11:35:48 (749 KB/s) - written to stdout [13]
```

Would it be possible to make them configurable?

For the time being, I've switched to another source with this change:

```
# diff /usr/lib/byobu/ip_address{.bak,}
44c44
< timeout 1 wget -q -O- http://v6.ipv6-test.com/api/myip.php </dev/null >"$cache" 2>/dev/null &
---
> timeout 1 wget -6 -q -O- http://ip.clara.net </dev/null >"$cache" 2>/dev/null &
63c63
< timeout 1 wget -q -O- http://v4.ipv6-test.com/api/myip.php </dev/null >"$cache" 2>/dev/null &
---
> timeout 1 wget -4 -q -O- http://ip.clara.net </dev/null >"$cache" 2>/dev/null &
68c68
< timeout 0.2 wget -q -O- http://169.254.169.254/latest/meta-data/public-ipv4 </dev/null >"$cache" 2>/dev/null &
---
> timeout 0.2 wget -4 -q -O- http://169.254.169.254/latest/meta-data/public-ipv4 </dev/null >"$cache" 2>/dev/null &

```

Note: that I've added -4 or -6 to the wget commands to make sure the proper protocol version is selected as the hostname is the same for both cases here.