RFC: 8139864: Improve handling of stack protection zones.

[Lindenmaier, Goetz]

Hi,

I opened this issue because I think the lower bounds for StackRedPages,
StackYellowPages and StackShadowPages introduced in 8078556 cause
problems on systems with page sizes > 8K.

For a more detailed description of the problems to expect see
https://bugs.openjdk.java.net/browse/JDK-8139864.

I see a row of possible solutions to this.

1.) Just set the lower bounds to '1'.  That's what I proposed before.
    I can imagine setups where one wants to reduce the size of these zones to a minimum.
    If an application starts a lot of threads, and is known not to overflow stacks, this
    might be desirable to save memory.

2.) Implement constraint methods that assure the zones never get smaller than DEFAULT_STACK_*_PAGES * default_page_size.
    This is necessary if you want to make sure the zones never can get smaller than that.

3.) Introduce new flags StackRedZoneSize, StackYellowZoneSize and StackShadowZoneSize.
    Set the default values to those of StackRedPages *4K etc, which is the most common page size I guess.
    During startup, round these to page size.
    Use Stack*ZoneSize in the code.
    If Stack*Pages flags are set on the command line, convert them to Stack*ZoneSize by multiplying with the
    current page size.

I'm happy to make a webrev for the favored solution.

Best regards,
  Goetz