helpers: make the proto field as a list rather than one option

The field proto in the struct fw3_cthelper should be implemented
as a list in order to support multiple protocols.

For example, the helper for SIP should be able to support both
TCP and UDP within only one entry in the config file.

config helper
        option name 'sip'
        option description 'SIP VoIP connection tracking'
        option module 'nf_conntrack_sip'
        option family 'any'
        option proto 'tcpudp'
        option port '5060'

Signed-off-by: Pierre Lebleu <pme.lebleu@gmail.com>