bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

[Stefan Monnier]

> Usually MAX_ALLOCA-related code falls back on malloc, and does not exit
> merely because a request was larger.  callproc.c's child_setup function is

MAX_ALLOCA is chosen small so that we can allocate several/many objects
of size MAX_ALLOCA.  Whereas in this case, after this alloca we're not
expected to use up the stack much more (we're about to `exec' anyway,
right?).  So MAX_ALLOCA is much too conservative for this case.


        Stefan