netlink: use consistent variable lifetime in the default snl(3) parsers.

Currently, parsers use original strings/nla pointers instead of
duplicating them. These pointers refer to the temporary packet buffer,
which can be silently rewritten when the next message is read.
Instead, duplicate all string/nla attributes using snl_allocz(3) to
give control over variable lifetime to the user.

MFC after: 2 weeks