MINOR: intops: add a multiply overflow detection for ulong and size_t

MINOR: intops: add a multiply overflow detection for ulong and size_t

Sometimes we'd like to know if some products overflow, so let's add a
pair of functions for this, for ulong and for size_t. For recent enough
compilers (gcc >= 5, clang >= 3.4) we just use __builtin_mul_overflow()
otherwise we rely on a division and a comparison before performing the
operation.

A third function, array_size_or_fail() computes the size of an array
of m elements of n bytes each, and returns the total size if it fits
in a size_t, otherwise ~0 if it does not so that passing this to
malloc() or any other variant would fail by trying to exhaust the
entire memory space.