force kresd to follow net.ipv(4,6) settings when forwarding

See merge request !710

Continuation of the parent commit.  In particular, kr_nsrep_set()
can't be used to create NS list "with holes".

The problem was the mm_malloc() typo :-/ so allocation was going through
malloc() instead of the packet's allocator (always memory pool ATM).
I added mm_free() for better correctness anyway.

- session: data length would be difference between start and end
  indices, but the function is unused so why even have it?

When decoding large packets, gnutls gives the application chunks
of size 16kb. So that tls session wirebuffer must be at least
KNOT_WIRE_MAX_PKTSIZE + 16kb.  (message re-formatted by vcunat)

Fixes #424.

Fixes #424.