Add .eml extension to archives files

[ldidry]

Fix #1581

[ikedas]

I think it is desirable to convert line breaks from <LF> to <CR><LF> in the .eml files to be included in the ZIP file.

It appears that newlines in .eml files would be better to be <CR><LF>. At least Firefox uses <CR><LF> in the .eml files it exports. Also, from this discussion, this seems to be the intended behavior. There may be some other applications that cannot open .eml files without <CR><LF> (Firefox itself, in recent versions, can open message files even if they use <LF>).

In contrast, many MTAs/MDAs on Unixen use native newlines when storing messages or passing messages to external programs: At least mbox (most of variants) and Maildir formats convert newlines to <LF>. That's why <LF> is also used for messages stored by Sympa.

[ikedas]

I agree to tag this PR "ready".