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