getty: don't accept ancient '#' and '@' as backspace/kill line,

getty: don't accept ancient '#' and '@' as backspace/kill line,
it only confuses people.
(Alexander Griesser <alexander.griesser@lkh-vil.or.at> (LKH Villach))
various other cleanups.

function                                             old     new   delta
getty_main                                          2526    2546     +20
static.baud_index                                      4       -      -4
parse_speeds                                          91       -     -91
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 1/0 up/down: 20/-95)            Total: -75 bytes
   text    data     bss     dec     hex filename
 773152    1086    9008  783246   bf38e busybox_old
 773081    1086    9008  783175   bf347 busybox_unstripped