bug#22910: read-only setlocale has side effect

[Zefram]

A call to setlocale with no second argument is documented to be
a read-only operation, querying the current locale configuration.
In fact it has a side effect of setting the encoding on primordial ports:

$ guile-2.0 -c '(write (port-encoding (current-input-port))) (newline) 
(setlocale LC_TIME) (write (port-encoding (current-input-port))) (newline)'
#f
"ANSI_X3.4-1968"

Observe that this occurs even if the locale reading operation is for a
category unrelated to character encoding.  The actual decoding behaviour
of read-char is altered in accordance with the reported encoding.
Non-primordial ports opened before or after the setlocale call are
not affected.

-zefram