Don't pass raw newlines when using --headers

[積丹尼 Dan Jacobson]

Let's say a mail has a hundred newlines in its Subject,

$ nl -b a
   101  Subject:
   102
   103
...
   267
   268   Your November
   269   Search performance for https://radioscanningtw.jidanni.org/
   270  From: Google Search Console Team <address@hidden>

$ hd

00001ae0  65 2e 63 6f 6d 3e 0a 53  75 62 6a 65 63 74 3a 20  |e.com>.Subject: |
00001af0  20 0a 20 0a 20 0a 20 0a  20 0a 20 0a 20 0a 20 0a  | . . . . . . . .|
*
00001c30  20 0a 20 0a 20 0a 20 0a  20 0a 20 0a 20 0a 20 59  | . . . . . . . Y|
00001c40  6f 75 72 20 4e 6f 76 65  6d 62 65 72 0a 20 53 65  |our November. Se|
00001c50  61 72 63 68 20 70 65 72  66 6f 72 6d 61 6e 63 65  |arch performance|

and also lines 268 and 269 starting with spaces.
Well, in --header summary, not only are lots of newlines
sent to the terminal, but the final two (268, 269) lines are already
beyond the buffer limit... so gone.

Mutt on the other hand treats this perfectly. All newlines and spaces
folded into one single line.

In fact no matter upon reading, or even replying, mutt doesn't bother
the reader with the newlines, and the reply Subject is properly folded
as if the big mess never existed.

(True, with mutt one might never know there was a problem.)

mail (GNU Mailutils) 3.7