knownhosts: Use ssh_mkdirs() instead of ssh_mkdir()
Previously, if the path to known_hosts file set through
SSH_OPTIONS_KNOWNHOSTS included missing directories,
ssh_session_update_known_hosts() would fail. The added test case checks
that this is not the case anymore.
The logic of checking if the directory is accessible before creating it
was replaced by creating the directory if opening the file failed. This
is to minimize the risk of TOCTOU race conditions.
Signed-off-by: Anderson Toshiyuki Sasaki <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>