Store authentication files only as user readable

As reported into silkaj#420 (closed), save v1 auth file as user readable 600 rw------- mode, not group and world readable.

I found two approaches:

Using the umask

Set 077 umask (for 600 permissions) and restore previous one:

current_umask = os.umask(0o077)
with open(path, encoding="utf-8") as fh:
  fh.write(seedhex)
os.umask(current_umask)

• Inspired from libnacl that we are already using.

Using an opener

def opener_user_rw(path, flags):
    return os.open(path, flags, 0o600)

with open(path, "w", encoding="utf-8", opener=self.opener_user_rw) as fh:
    fh.write(seedhex)

• https://docs.python.org/3/library/functions.html#open
• https://stackoverflow.com/a/45368120
• https://github.com/python/cpython/issues/73400#issuecomment-1093736938