It's better to start a test HTTP server or override `@curl` from tests
in `t/*.pl`.

This makes log messages easier to understand.

This is simpler, and makes it possible to distinguish unset from set
to an empty string.

The same message is already logged in `get_ipv4`.

Why issue a warning that things have suddenly started going well?
It's bizarre.

Before, the first line of a multi-line log message was prefixed with a
space while all subsequent messages were prefixed with `|`.  Now the
first line is prefixed with `>` and all subsequent lines with a space.
This makes it easier to quickly discern message boundaries.

The non-destructive substitution modifier wasn't added until Perl
v5.14.0.

`geturl` already debug logs the response.

`header_ok` already checks for non-2xx codes.

`header_ok` already checks whether the response is `undef`.

for readability.

It's already initialized to a non-success value.

This also makes it possible to change "ipv4" and "ipv6" in log
messages to "IPv4" and "IPv6".

It's already initialized to a non-success value.