reimplement if_nameindex and getifaddrs using netlink

the previous implementations had several deficiencies, the most severe
of which was the inability to report unconfigured interfaces or
interfaces without ipv4 addresses. among the options discussed for
fixing this, using netlink turned out to be the one with the least
cost and most additional advantages. other improvements include:

if_nameindex now avoids duplicates in the list it produces, but still
includes legacy-style interface aliases if any are in use.

getifaddrs now reports hardware addresses and includes the scope_id
for link-local ipv6 addresses in the resulting address.

include cleanups: remove unused headers and add feature test macros

fix missing SOCK_CLOEXEC in various functions that use sockets internally

fix logic error for skipping failed interfaces in if_nameindex

implement if_nameindex and if_freenameindex