bug#33612: Gnus doesn’t display EBCDIC encoded mails correctly

[Thomas Schneider <qsx@chaotikum.eu>]

If you read this subject and ask yourself “What in the name of—?!”, yes,
exactly.  This is cursed and should really not be a real-world issue,
but I still consider it a bug.

When viewing a mail whose headers and/or body is EBCDIC encoded, Gnus
does not decode it correctly.

Given this mail …

```
From: =?EBCDIC-INT?B?44iWlIGiQOKDiJWFiYSFmQ==?= <qsx@qsx.re>
To: =?EBCDIC-INT?B?44iWlIGiQOKDiJWFiYSFmQ==?= <qsx@qsx.re>
Date: Tue, 04 Dec 2018 16:55:15 +0100
Subject: =?EBCDIC-INT?B?44iJokCipIKRhYOjQImiQImVQMXCw8TJww==?=
Content-Type: text/plain; charset="EBCDIC-INT"
Content-Transfer-Encoding: base64
Message-ID: <F7A3S3POYB.3P9ZHF8WLUSKQ@coruscant.chaotikum.eu>

44iFQJikiYOSQIKZlqaVQIaWp0CRpJSXokCWpYWZQKOIhUCTgamoQISWh0sl
```

the Summary buffer shows (with gnus-summary-line-format "%U%R
%&user-date; %(%[%5k: %-23,23f%]%)%B%s\n") …

```
 .           T16:55:15 [ 1.0k: =?EBCDIC-INT?B?44iWlIGi]  =?EBCDIC-INT?B?44iJokCipIKRhYOjQImiQImVQMXCw8TJww==?=
```

and the Article buffer shows …

```
From: =?EBCDIC-INT?B?44iWlIGiQOKDiJWFiYSFmQ==?= <qsx@qsx.re>
Sender: qsx@chaotikum.eu
To: =?EBCDIC-INT?B?44iWlIGiQOKDiJWFiYSFmQ==?= <qsx@qsx.re>
Subject: =?EBCDIC-INT?B?44iJokCipIKRhYOjQImiQImVQMXCw8TJww==?=
Date: Tue, 04 Dec 2018 16:55:15 +0100 (32 minutes, 23 seconds ago)
Message-ID: <F7A3S3POYB.3P9ZHF8WLUSKQ@coruscant.chaotikum.eu>

\343\210\205@\230\244\211\203\222@\202\231\226\246\225@\206\226\247@\221\244\224\227\242@\226\245\205\231@\243\210\205@\223\201\251\250@\204\226\207K%
```

This was copy-pasted from a terminal Emacs, it shows the raw bytes
escaped as octal numbers.

So while the decoding routines work fine on more common charsets like
UTF-8 and ISO 8859-1, they fail on EBCDIC.  Not something likely to be
encountered in real life, but still.


Gnus v5.13
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.1)
 of 2018-10-25