[zx] Migrate out-param handles to use reset_and_get_address().

Migrates two remaining call-sites to use reset_and_get_address(),
and removes a TODO for moving more APIs to the relevant object<>
specialisations, in favour of the current checked-by-traits
approach.

TEST=CQ
Bug: ZX-2310
Change-Id: I7a6946f6ed3374b5ba883e82fe32110fa6029acb

[zx] Move public zx headers to match naming convention

The zx library provides headers intended for general use, so the
include path should be spelled <lib/zx/foo.h> as per
https://fuchsia.googlesource.com/docs/+/master/development/languages/c-cpp/naming.md.
This moves the headers into the proper location and updates includes to
match.

Change-Id: I6fc9b2a491b5f8b0d270c4dfc77fa4be5739071b

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94