Add support for IFA_FLAGS

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.
Merge pull request #68 from vodik/IFA_FLAGS

IFA_FLAGS support