#1 Allow to remeber default choice and adds a timeout

[Gael Le Mignot]

This patch allows to remember the option chosen in the
netenv menu, and use it as default on the next boot.
Default is remembered if NETENV_SAVEDEFAULT is set in
the configuration file for the selected item.

A global (in the netenv script) option adding a timeout
is present too.

This is very usefull when the laptop is moved from
location to location on relatively low frequency, and
sometimes rebooted from ssh (since I've a dual boot
between GNU/Linux and GNU/Hurd)

[Gael Le Mignot]

The patch