Mark Andrews authored May 28, 2020 and Michał Kępień committed Jun 18, 2020

NS_CLIENT_TCP_BUFFER_SIZE was 2 byte too large following the
move to netmgr add associated changes to lib/ns/client.c and
as a result an INSIST could be trigger if the DNS message being
constructed had a checkpoint stage that fell in those two extra
bytes.  Adjusted NS_CLIENT_TCP_BUFFER_SIZE and cleaned up
client_allocsendbuf now that the previously reserved 2 bytes
are no longer used.