menuconfig: remember alternate config filename

When loading an alternate configuration use that file as
current configuration filename.
Make the filename visible in the dialog.
Default continue to be .config.

Inspired by patch from: Cyrill Gorcunov <gorcunov@gmail.com>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc:  Cyrill Gorcunov <gorcunov@gmail.com>