ash: use bbox wrappers for malloc etc instead of homegrown ones

ash: use bbox wrappers for malloc etc instead of homegrown ones

function                                             old     new   delta
popstring                                            134     140      +6
ckmalloc                                               9       -      -9
ckstrdup                                              22       -     -22
ckrealloc                                             24       -     -24
ckzalloc                                              28       -     -28
------------------------------------------------------------------------------
(add/remove: 0/4 grow/shrink: 1/0 up/down: 6/-83)             Total: -77 bytes

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