askrene: fix integer overflow for dummy channels

[Lagrang3]

We may introduce high capacity channels in askrene to represent problems
with multiple destinations (eg. multiple blinded paths) or to solve
self-payments. The integer computation of the deliverable amount through
these channels (I tested this with a channel with 21M bitcoin) would
fail due to an integer overflow in the function amount_msat_sub_fee,
21M BTC = 21 x 10^17 msat, which overflows u64 integers when multiplied
by 10^6. We have fixed amount_msat_sub_fee operations, so that it
doesn't overflow.

Basically we had the following integer operation:

floor(x * a/b)

where a<b and a=10^6, so the result fits in u64. The problem is that we first compute a*x and then divide by b.
a*x can overflow.
We fixed the operation by splitting x = q_x * b + r_x where q_x = floor(x/b) and r_x = x mod b, so that
our computation reduces to:

a * q_x + floor(a*r_x / b)

The value a*q_x <= x does not overflow. So the only dangerous operation is a*r_x,
but a*r_x < a*b, so if a*b does not overflow then the entire operation is safe to compute.
In our case a=10^6 and b = 10^6 + ppm, so unless ppm ~ 10^12, which is an absurd value,
the above operation is guaranteed to be successful.

[Lagrang3]

Optionally, we can use the same integer decomposition to compute amount_msat_fee.

[rustyrussell]

Ack e0004fa