acme client: fix #3536 untaint data returned from acme server

acme client: fix #3536 untaint data returned from acme server

The data returned from the acme server (e.g. boulder) should be
considered tainted.

To see which places might need untainted I checked where $self->{ua}
was used (in $self->do) and a quick scan identified __get_result as
the consumer of the tainted data.
In all but one uses the data is decoded from json (which would die if the
result is not valid json).

The remaining use-case yields a certificate in PEM format (and is
handled at the caller of __get_result).

The issue is currently only visible if a proxy is set, because AFAICT
somewhere in SSLeay (or IO::Socket::SSL, which uses SSLeay) a taint
flag is not set on the return value.

A reproducer for the issue:
```

use strict;
use warnings;

use HTTP::Request;
use LWP::UserAgent;

$ENV{PATH} = "/usr/bin:/bin";

my $ua = LWP::UserAgent->new(env_proxy => 0);
my $request = HTTP::Request->new('GET', 'https://google.com/');
my $resp = $ua->request($request);
my $text = substr($resp->decoded_content, 0, 5);;
system("echo \"$text\""); # does work
$request = HTTP::Request->new('GET', 'http://neverssl.com/');
$resp = $ua->request($request);
$text = substr($resp->decoded_content, 0, 5);;
system("echo \"$text\""); # does not work
```

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>