trafgen: parser: Add syntax to generate DNS header

trafgen: parser: Add syntax to generate DNS header

Add new syntax for DNS header generation via 'dns()' proto function.

The fields are supported:

	id		- 16 bit identifier
	qr		- message is a query(0) or response(1)
	op|oper		- specified kind of query
	aanswer		- authoritative answer flag
	trunc		- message was truncated flag
	rdesired	- recursion desired flag
	ravail		- recursion available flag
	zero		- reserved for future use
	rcode		- response code
	qdcount		- number of entries in question section
	ancount		- number of entries in answer section
	nscount		- number of entries in authority section
	arcount		- number of entries in additional section

Also there are functions to generate DNS sections:

'qry()' function to generate separate query entry:

	name		- variable domain name
	type		- type of the query
	class		- class of the query

'ans()', 'auth()', 'add'  functions to generate separate answer,
authoritative, adidditional entry with the same fields layout:

	name		- variable domain name
	type		- resource record type
	class		- class of the data
	ttl		- time interval that the record may be cached
	len		- length of data
	data		- variable length of bytes

All the DNS section entries will be automaticlly sorted by DNS proto API
in the way which is required by DNS header:

	query entries
	answer entries
	authoritative entries
	additional entries

'name' field in qry/ans/auth/add functions is automatically converted to
FQDN format if it was specified as "string".

There are also added functions to simplify the way of filling
some often used RR types for using them inside ans/auth/add functions:

	addr(ipv4_addr | ipv6_addr) - fills the following RR fields:
		len  - 4 or 16 depends on IPv4 or IPv6 address was specified
		data - is filled with IPv4 or IPv6 address
		type - 1 for IPv4 address, 28 - for IPv6

	ns(string)
		type - 2

	cname(string)
		type - 5

	ptr(string)
		type - 12

EXAMPLES:

{
    dns(qr=1,
        auth(name="ns1", ns("ns1.org")),
	ans(name="www.google.com", cname("google.com")),
	auth(name="aa", ns("bb")),
	qry(name="www.google.com"))
}

{
    dns(qr=1, ans(name="www.google.com", addr(1.2.3.4)))
}

{
    dns(qr=1, ans(name="www.google.com", addr(1::)))
}

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