chardev: Reduce "char-mux.h" scope, rename it "chardev-internal.h"

No file out of chardev/ requires access to this header,
restrict its scope.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200423202112.644-5-philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>