Improve the function ``cleanup_filename()`` of ``lib/mu-msg-part.c`` to
use Unicode characters when replacing the control characters, slashes
and colons with ``-``.
Originally, this function just use plain C characters (i.e., assuming
ASCII string) when checking each character is or not a control character,
slash or colon. However, when the attachment filename contains non-ASCII
(e.g., Chinese characters), all the non-ASCII characters are replaced
with ``-``.
For example:
* Before:
```
> mu view test_chinese_attachment_filename.eml
From: Tester <tester@example.com>
To: Example <example@example.com>
Subject: Test email with attachment of Chinese filename
Date: Mon 23 May 2016 05:22:09 PM CST
Attachments: 'attachment-test.txt', '------------.txt', '-------test.txt'
Hello,
This is a simple test email with three attachments:
1. `attachment:test.txt`: filename is all English;
2. `测试附件.txt`: filename is all Chinese (exclude the extension);
3. `附件-test.txt`: filename mixes Chinese and English.
```
* After:
```
> ./build/mu/mu/mu view test_chinese_attachment_filename.eml
From: Tester <tester@example.com>
To: Example <example@example.com>
Subject: Test email with attachment of Chinese filename
Date: Mon 23 May 2016 05:22:09 PM CST
Attachments: 'attachment-test.txt', '测试附件.txt', '附件-test.txt'
Hello,
This is a simple test email with three attachments:
1. `attachment:test.txt`: filename is all English;
2. `测试附件.txt`: filename is all Chinese (exclude the extension);
3. `附件-test.txt`: filename mixes Chinese and English.
```
297df938d6