SFTPFS: verify server fingerprint (fix CVE-2021-36370).

SFTPFS: verify server fingerprint (fix CVE-2021-36370).

Use ~/.ssh/known_hosts file to verify server fingerprint
using ssh way:

$ ssh localhost
The authenticity of host 'localhost (127.0.0.1)' can't be established.
ED25519 key fingerprint is SHA256:FzqKTNTroFuNUj1wUzSeV2x/1lpcESnT0ZRCmq5H6o8.
Are you sure you want to continue connecting (yes/no)? no
ssh: Host key verification failed.

$ ssh localhost
The authenticity of host 'localhost (127.0.0.1)' can't be established.
ED25519 key fingerprint is SHA256:FzqKTNTroFuNUj1wUzSeV2x/1lpcESnT0ZRCmq5H6o8.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'localhost' (ED25519) to the list of known hosts.
andrew@localhost's password:

Thanks the Curl project for the used code.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>