Validate G1 options that are expressed as percentages

There are a number of G1-specific options that are expressed as percentages, but which are not validated during argument processing. In some cases there is no validation at all, and providing arguments outside [0,100] may produce unexpected behavior or crashes. In other cases there is some limited checking and canonicalization where the option is used, usually with a warning message for invalid values. It would be better to always validate these options during argument processing.

Specific options include:
- G1SATBBufferEnqueueingThresholdPercent
- G1RSetUpdatingPauseTimePercent
- G1HeapWastePercent
- G1ReservePerscent