Process dig -x reverse octets iteratively

Process dig -x reverse octets iteratively

reverse_octets() recursed once per dot, with depth bounded only by
ARG_MAX (~2 MiB on Linux), so feeding dig -x a deep input like
'1.1.1.…1' busted the call stack and crashed the tool with SIGSEGV
instead of a structured error.  The transformation it performs is
purely textual (split on '.', emit components in reverse), so the
recursion was never load-bearing.

Walk the input once into a fixed-size array of label slices, capped at
DNS_NAME_MAXLABELS (which is the most we could ever fit into the
result buffer anyway), then iterate the array in reverse to write the
output.  Inputs with more than DNS_NAME_MAXLABELS labels now return
DNS_R_NAMETOOLONG, which dig.c surfaces as 'Invalid IP address' and
exit 1.  Drop the unnecessary (int) casts on ptrdiff_t/size_t lengths
while at it.

Assisted-by: Claude:claude-opus-4-7