Adjust signature of walrcv_receive hook.

Commit 314cbfc5 redefined the signature of this hook as
typedef int (*walrcv_receive_type) (char **buffer, int *wait_fd);

But in fact the type of the "wait_fd" variable ought to be pgsocket,
which is what WaitLatchOrSocket expects, and which is necessary if
we want to be able to assign PGINVALID_SOCKET to it on Windows.
So fix that.