Add optional per-chain byte cap in chained buffers

Add optional per-chain byte cap in chained buffers

Previously, njs_chb_t accumulated bytes unbounded through the memory
pool until the final string boundary (njs_string_create_chb()) enforced
the NJS_STRING_MAX_LENGTH limit.  For string-producing chains this let
the pool grow to tens of GiB before the final alloc was refused, which
on adversarial inputs (such as String.prototype.replace with a regex
that matches the full string and a replacement containing thousands of
$N references) led to process OOM instead of a catchable RangeError.

The fix is to add two fields to njs_chb_t: total_size (O(1) running byte
count) and max_size (optional upper bound, 0 means unlimited).
Exceeding max_size puts the chain into a sticky NJS_CHB_ERR_OVERFLOW
state.  As part of the change njs_chb_drain() now returns early on a
failed chain, in line with njs_chb_drop().