trafgen: parser: Allow to set function at field offset

trafgen: parser: Allow to set function at field offset

Extend proto field expression to:

    proto_field[{index}:{len}] = {func}

which allows to specify function on the field offset via index
and value length (default is 1 - 1 byte). This rule is optional.

It was needed to keep of proto_field's copies in packet_dyn->fields
instead of original fields which allows to scpecify different functions
on the different parts of same field, also the copy of original
proto_field allows to set custom length/pkt_offset which makes such
field behave as virtual sub-field of the original one with different
length/pkt_offset but point to the same piece of header.

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