bug#35920: strftime incorrectly assumes that nstrftime will produce UTF-8

[Mark H Weaver]

There might also be related problems with 'strptime'.  These problems
date back to when Guile was first extended to support non-ASCII strings.
Here's the relevant commit in 2009 that added non-ASCII support to
'strftime' and 'strptime', but did so imperfectly:
587a33556fdef90025c1b7d4d172af649c8ebba8

       Mark