[fdio] Initial implementation of getifaddrs

Sets up the API plumbing and implement getifaddrs to the point where a
list of all addresses are returned.

Due to lack of support in the current version of the netstack,
broadcast address (missing IFF_BROADCAST flag) and destination address
are not set. They will be added when corresponding support is added in
the netstack.

Support for AF_PACKET addresses will be added separately.

Test:
fx test -d getifaddrs-netemul-test
fx test --host getifaddrs-test

Bug: 30719, 54162, 54163

Change-Id: I52910b5bbf5a4d493fd9b591e1dbb2203aa51ef9
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/398773
Commit-Queue: Jay Zhuang <jayzhuang@google.com>
Reviewed-by: Tamir Duberstein <tamird@google.com>
Reviewed-by: Adam Barth <abarth@google.com>
Reviewed-by: Chris Tam <godtamit@google.com>
Testability-Review: Tamir Duberstein <tamird@google.com>
API-Review: Adam Barth <abarth@google.com>