trafgen: Add basic protocol generation logic

trafgen: Add basic protocol generation logic

Add new trafgen_proto.c module with basic protocol header fields
generation logic. This will allow to support protocol specific keywords
in the trafgen configuration language.

Each protocol must implement struct proto_hdr and register it to the
global proto list.

Protocol headers consist of a set of fields, and each field must be
described via struct proto_field by specifying unique id, length and
offset (relative to the header start). Fields smaller than 8 bits can be
described via left shift & mask.

The following callbacks are invoked to perform special actions to build
the header during parsing:

    1) header_init - required fields must be added to the packet and
        initialized with default values.

    2) header_finish - it is invoked when header is specified, all user
        specified fields are set.

    3) packet_finish - callback is invoked from upper to lower header
	to calculate fields depending on upper layers such as total
	length or checksum.

The protocol generation API provides convenience protocol field
setters/getters to to be used in the parser while crafting the packet.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
[tk: wordsmithing on commit message, minor variable type changes]
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>