Spotted another opportunity to use reallocarray().
ok miod
handle the (impossible) situation of a size_t - 1 buffer from
EC_POINT_point2oct so that later allocation does not overflow
with miod
We're not supporting 16-bit Windows, remove cast.

ok jsing@ deraadt@
Move Windows OS-specific functions to make porting easier.

Several functions that need to be redefined for a Windows port are right
in the middle of other code that is relatively portable. This patch
isolates the functions that need Windows-specific implementations so
they can be built conditionally in the portable tree.

ok jsing@ deraadt@