SFTP: the lines of the shell session error and df output should end in CRLF

[davidsarah]

Current result of attempting a shell session to the SFTP frontend:

$ ssh -p 8022 davidsarah@127.0.0.1
davidsarah@127.0.0.1's password: 
This server supports only SFTP, not shell sessions.
                                                   Connection to 127.0.0.1 closed.

Expected result:

$ ssh -p 8022 davidsarah@127.0.0.1
davidsarah@127.0.0.1's password: 
This server supports only SFTP, not shell sessions.
Connection to 127.0.0.1 closed.

[davidsarah]

SFTP: Make the lines of the shell session error and the df output end in CRLF. fixes #1446

[davidsarah]

I manually checked that the new df output is still accepted by sshfs and OpenSSH's sftp.

[kevan]

I posted my review comments in comment 10 of #1442, since the patch attached to that ticket also included the patch to this ticket.

[david-sarah@…]

In a2699ea6f635c992:

SFTP: write an error message to standard error for unrecognized shell commands. Change the existing message for shell sessions to be written to standard error, and refactor some duplicated code. Also change the lines of the error messages to end in CRLF, and take into account Kevan's review comments. fixes #1442, #1446