remote: fix logic for known_hosts and keyfile checks
    
    If any of them is specified for the libssh and libssh2 drivers, there is
    no need to depend on checks based on other paths: in particular, a
    specified path for known_hosts was ignored if the local config directory
    could not be determined, and the path for keyfile was ignored if the
    home could not be determined.
    
    Instead, lazily determine and use these two paths only in case they are
    needed.