[zx] Clean up some libzx container APIs, implementations, and callers.

- Add a zx::job::create() API accepting zx::job rather than zx_handle_t.
- Migrate all out-param handles to be output via reset_and_get_address().

Bug: ZX-2310
TEST=libzx-test
Change-Id: Ib9cb9a0778d8747f35996134038936d1f7bb3397

[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