Improve netlink debugging.

1. When attributes are nested, don't print them when decoding;
   they will be printed when the contining attribute is printed.
2. Support decoding a couple more interface attributes.
3. Support decoding XFRM_MSG_NEWPOLICY messages.

Test: all_tests.sh passes on android-4.9
Change-Id: I2828a9dcfe4de4358bada0b0a679625d552b538f