Before this change rclone about was failing with

    cannot unmarshal number 1.0e+18 into Go struct field User.space_amount of type int64

As Box increased Enterprise accounts user.space_amount from 30PB to
1e+18 or 888.178PB returning it as a floating point number, not an integer.

This fix reads it as a float64 and clips it to the maximum value of an
int64 if necessary.

Before this fix it was possible for an about call in various backends
to exceed an int64 and wrap.

This patch causes it to clip to the max int64 value instead.