trafgen: parser: Add syntax for IPv4 protocol generation

trafgen: parser: Add syntax for IPv4 protocol generation

Add 'ip4(), ipv4()' function to build IPv4 header.

Fields supported:

	ihl		IPv4 header length (default 5 )
	ver|version	IPv4 version (default 4)
	ttl		Time To Live (TTL) field (default 0)
	dscp		DiffServ field (default 0)
	ecn		ECN bits (default 0)
	tos		TOS (DSCP + ECN) field (default 0)
	len|length	IPv4 header + payload length (calculated by default)
	id		Identifier (default 0)
	flags		Flags field (default 0)
	frag		Fragment offset (default 0)
	csum		IPv4 header checksum (calculated by default)
	df		Set DF (Dont Fragment) bit to 1 (default is 0)
	mf		Set MF (More Fragments) bit to 1 (default is 0)
	sa|saddr        IPv4 source address (default used from output device)
	da|daddr	IPv4 destination address (default 0.0.0.0)
	proto		IPv4 protocol id (default 0)

Example:

    { ip4(df, mf, frag=100, prot=0x1, ecn=2, dscp=20) }
    { ip4(da=1.1.1.1), ip4(sa=3.3.3.3, da=3.3.3.4) }

Last example generates IP-in-IP packet.

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