SFTP backend fails when connecting with the "Timed out when logging in"
error with the recent OpenSSH versions. This is because their password
prompts are now prefixed by the `(user@host)` string, which breaks our
`g_str_has_prefix` checks. Let's use `strstr` instead to fix this issue
and also to not break compatibility with old versions and other vendors.

Fixes: GNOME/gvfs#584