IFA_FLAGS support

`IFA_FLAGS` is a u32 attribute that extends the u8 field `ifa_flags`. If present, the value from `struct ifaddrmsg` will be ignored.

Currently two flags are stored in IFA_FLAGS: `IFA_F_MANAGETEMPADDR` and `IFA_F_NOPREFIXROUTE` which are useful for ipv6 SLAAC support.