slattach: code shrink, better --help text

slattach: code shrink, better --help text

function                                             old     new   delta
tcsetattr_serial_or_warn                               -      34     +34
static.int_N_SLIP                                      -       4      +4
restore_state_and_exit                               123     117      -6
packed_usage                                       31774   31747     -27
set_termios_state_or_warn                             42       -     -42
slattach_main                                        673     624     -49
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/3 up/down: 38/-124)           Total: -86 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>