shell/math.c: rename arith_eval_hooks to arith_state, put error code into it

shell/math.c: rename arith_eval_hooks to arith_state, put error code into it

function                                             old     new   delta
expand_and_evaluate_arith                             79      89     +10
arith                                                675     674      -1
arith_lookup_val                                     151     142      -9
ash_arith                                            135     122     -13
arith_apply                                         1304    1269     -35
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/4 up/down: 10/-58)            Total: -48 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>