os::split_reserved_memory() should be cleaned up a bit.

It is used to split reserved memory regions in order to make them independent units. This only matters when releasing them, so after splitting these regions should be releasable independently from each other.

This only matters on Windows, which is the only platform with a non-empty implementation, since virtual memory can only be released as a unit (opposed to mmap api, where sub regions can be unmapped individually).

Improvements:
- the interface should be commented
- the "realloc" parameter can be removed. It has never not been true on all code paths - had it been, it would have been an error on windows resulting in loosing one of the two sides of the split.
- the many non-windows implementations can be combined in posix.cpp.