Initial attempt at adding multiple pd to scripts

[jrmithdobbs]

This adds all pds besides zero with their identifier as a base10 int after
PDINFO in the variable name, a PDCOUNT variable, as well as related PDIF with a
space separated list of {interface}:{sla_len}:{sla_id} following same naming as
PDINFO, a CDUID (client) and SDUID (server) so that this information can be
passed to other systems.

For example where the second pd has no interfaces:

SDUID=<serverDUID>
CDUID=<interfaceDUID>
PDINFO=1111:1111:1111::/56
PDIF=vlan1:8:0 vlan2:8:2 vlan3:8:3
PDINFO1=1111:1111:1112::/56
PDINFO2=1111:1111:1113::/56
PDIF2=vlan4:8:0
PDCOUNT=3

This addresses (and then some) #19.

[jrmithdobbs]

The majority of these changes apply to the REPLY to the initial REQUEST if there are multiple PDs being requested/offered.

It would be nice if the address decided to be assigned was also available in the script but I’ve not read through enough of dhcp6c_ia.c to tell where/if that’s being stored or what needs to be plumbed to make it available here.

The sla_len and sla_id values combined with the knowledge that it uses the link-local address for the postfix at least make it derivable. Maybe the separator shouldn’t be : so that the assigned address could be added later?

[fichtner]

@jrmithdobbs if you still read this and are interested. I've chosen the smallest approach here to inject all prefixes into PDINFO variable only. I don't mind adding SDUID and CDUID per se, but it's a bit of a side quest as well as restructuring the information given here since we would start one script per interface anyway which makes PDINFO scope local. Otherwise we would probably have to emit the interface as well (not even counting internal vs. external reference name in OPNsense). Happy about further PRs.

Cheers,
Franco