Instead of lying and claiming we always have space left in our receive
buffer, actually report the available space.
While this doesn't really affect network-bound workloads, it makes a
world of difference in cpu/disk-bound ones, like git clones. Resulting
in a considerable speed-up, and in some cases making them work at all.
(instead of the sender side hanging up the connection due to timeouts)
2c51ff763b