luci-base: support ip6ifaceid option for proto_static

Add support for 'ip6ifaceid' option for proto_static in LuCI.

Information about the option:
 The option is optional and defaults to '::1'.
 Allowed values: 'eui64', 'random', fixed value like '::1' or '::1:2'

 When IPv6 prefix (like 'a:b:c:d::') is received from a delegating
 server, the ip6ifaceid suffix (like '::1') is used to form
 the IPv6 address ('a:b:c:d::1') for the interface.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>