trafgen: l7: Add DNS header generation API

trafgen: l7: Add DNS header generation API

Add trafgen_l7.c module with DNS proto header generation with
support of filling DNS query/answer/authority/additional sections
as sub headers.

Introcuded new concept as 'sub header' which is needed to easy handle
DNS sections which might be added on-demand, and to simplify using
sub-header as regular header with a fields, offset, etc. There is a
parent header which contains array of pointers of sub-headers, and the
array is ordered as they are located in the parent header. The
sub-headers mostly encapsulated by the parent header which 'knows'
the semantic of them. The new proto_hdr->push_sub_header(...) callback
was added to tell the parent header to push the sub-header's fields,
sub-header also may have proto_ops which must be filled by the parent.
This sub-header concept might be used in the future if it will be needed
to support DHCP, WLAN headers.

There are 4 kinds of DNS sub-headers - query, answer, authority,
additional. 'id' of each sub-header is used to only differentiate these
types of sections. These sections have strict order inside DNS header,
and there was added the proto_hdr_move_sub_header(...) to sort them in
required order.

Actually there are only 2 proto_hdr's which describes 4 DNS sections -
query & rrecord, because rrecord covers another 3 - answer, auhority,
additional which have the same layout.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>