RFR(xs): 8245035: Clean up os::split_reserved_memory()

[Thomas Stüfe]

Hi,

may I have reviews for this small cleanup please:

jbs: https://bugs.openjdk.java.net/browse/JDK-8245035
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8245035--clean-up-os--split_reserved_memory()/webrev.00/webrev/

I need this as a preparation for
https://bugs.openjdk.java.net/browse/JDK-8243535, which I'd like to review
separately.

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 whole thing 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.

Thank you,

Thomas