Set accept_ra_rt_info_max_plen to 64 before enabling IPv6 interfaces

This patch tries to change accept_ra_rt_info_max_plen to 64 from its
default value of 0 on kernels that support router advertisements with
route information options. The code fails silently on older unsupported
platforms.

Also factor existing startIPv6() code into a self documenting function
enableInterfaceIpv6PrivacyExtensions()

Bug: 33333670
Test: as follows
    - build sailfish
    - flashed
    - booted
    - runtest frameworks-net passes
    - runtest frameworks-wifi passes

Change-Id: I10ec83b9e04836089477d74417b7222499820066