bug#22910: read-only setlocale has side effect

[Zefram]

Additional information: setlocale's side effect on primordial ports
happens even if the port's encoding has been individually set using
set-port-encoding!.  This means that to maintain a specific encoding on
these ports (other than the locale's nominal encoding, which is likely to
not be binary compatible) it is necessary to set the encoding repeatedly,
before any I/O operation after setlocale might have been called.
Since the read-only mode of setlocale has this effect, and arbitrary
library code might feel entitled to call setlocale for read purposes
without documenting that it does so, this really amounts to setting the
encoding before every I/O operation.

-zefram