generalize $RANDOM impl to support dynamic vars like SHELLOPTS + BASHOPTS

[reubeno]

I think it's worth looking at generalizing what we already do for $RANDOM -- i.e., the existence of a special variable whose reads are satisfied by custom functions. We could have some way to provide dynamically accurate values of SHELLOPTS, for example, without needing the code managing options to remember to update the variable. As a side benefit, this also allows us to do less work at shell initialization time to compute and format the right strings.