(fs) FileStore should support file stores with > Long.MAX_VALUE capacity

Summary

Make java.nio.file.FileStore.getTotalSpace and analogous methods return Long.MAX_VALUE instead of a negative value when the size of the space exceeds the range of a long.

Problem

Currently if the size of the type of space they correspond to exceeds the maximum value of a long, the methods getTotalSpace, getUnallocatedSpace, and getUsableSpace return a negative value. This is not helpful as it is ambiguous as to whether an overflow occurred or there was some other error.

Solution

Make the aforementioned methods return Long.MAX_VALUE if the size of the spaces they correspond to overflows a long.

Specification

The essence of the specification change is to add a sentence similar to this

    * If the number of unallocated bytes is greater than
    * {@link Long#MAX_VALUE}, then {@code Long.MAX_VALUE} will be returned.

to the specifications of the getTotalSpace, getUnallocatedSpace, and getUsableSpace methods of java.nio.file.FileStore. The complete changes are in the attached webrev archive.