fix NTP server option handling

previously, only option 56 (NTP server) was supported, but named SNTP_*
throughout code and in the scripts environment. This patch fixes this and adds
optnion 31 (SNTP Servers) support. Uses NTP_IP and NTP_FQDN and SNTP_IP in
script environment.

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories
added command line switch for strict oro

Now "-R" command line switch makes odhcpv6c only request those options
specified by "-r".

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories
add support for multiple prefixes with distinct IAIDs

Changes from v1:
- removed some unneeded changes
- use *_add_state instead of (semantically identical and so unnecessary)
  *_append_state

This is missing IAID validation for prefixes.

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories
allow disabling "client fqdn" and "accept reconfiguration" options

Some DHCPv6 servers require these to be absent, so add
command line switches that allow disabling them.

(includes "Combination of command line switches -f and -a
 broke "-N none" and IA_PD." from Christian Carstensen)

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories
add support Vendor Class option

This patch adds cli option "-V <hex-string>" which will enable sending
of DHCPv6 option 16 ("vendor class").

Based on Patch by Christian Carstensen

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories
output prefix IAID as prefix class in script

Contributed by T-Labs, Deutsche Telekom Innovation Laboratories