Increase the range of characters that get substituted by '_' so the
shell doesn't complain about illegal variable names.

Primarily done to catch '.' and '-' but who knows what funnies will
appear in i/f names.

It's a shame that busybox ash doesn't understand :alnum:

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>