Take public keys at newline, instead of on EOF

[Nick]

Hello, I have a suggestion in the attached patch. I hope you will consider it.

Thank you :)

[Sitaram Chamarty]

On Sun, Sep 19, 2021 at 11:56:34PM +0000, Nick wrote:
> Hello, I have a suggestion in the attached patch. I hope you will consider it.

I'd prefer to have the safe part still around.

Since this is an extremely rarely used feature, and requires
manual input anyway, I was thinking you could forget about the
efficiency of reading in chunks of 4096 till eof, and instead
read a single character at a time until a newline appears, while
also sticking to a limit of 4096.

Perhaps you could try that? Would involve a loop inside
safe_stdin(), and the main routine would not need any changes.

sitaram