shadowsocks-libev: ss-rules: add ipv6 support

shadowsocks-libev: ss-rules: add ipv6 support

It will require support from ip6tables-mod-nat.  The added functionality
will be skipped otherwise.

For $o_dst_bypass6_, include only address blocks in link [1] whose
"Globally Reachable" field are explicitly "False"

Closes openwrt/packages#7508

 [1] IANA IPv6 Special-Purpose Address Registry,
     https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

Signed-off-by: Yousong Zhou <redacted>