Since srfi-6 string ports are builtin to guile, should srfi-6 be
always true in cond-expand?  Ie. true before doing any use-module?

I don't suppose it makes much difference, but if the feature is
available it may as well be indicated.

        * boot-9.scm (%cond-expand-features): Add srfi-6 which is in the core.

(This could be for the 1.6 branch too.)