This was strongly recommended by Russell O'Connor: the "ms" implies that
it's a BIP-32 master secret, and this is CLN specific.
If we changed the hrp to "cln" it would be better, but apparently that
means we no longer fit in a "standard billfold metal wallet" (and
our code assumes a 2-byte prefix anyway).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>