[zx] Migrate libzx off legacy_unowned<T> and update call-sites.

- Migrate APIs like zx::process::self() from returning a
|const zx::legacy_unowned<T>| to returning |zx::unowned<T>|.
- Update callers of those APIs to use the appropriate dereference
operator to access members of T, or to get a |const T&| to pass to
libzx-wrapped syscalls.
- Remove the zx::legacy_unowned<T> template.

Bug: ZX-2310
TEST=CQ
Change-Id: Idd4267cd0246ec9ab0e6f1392efe7fda45ff169d

[zx] Add a zx::thread::start convenience overload.

This overload accepts pointers in a way that is normally used for
in-process threads.

Updates some callers to use the new function.

Clang-format thread.h

Change-Id: Ie3267604bb587b9473f749d47b987df10a9c0a1b

[syscalls] Remove count arguments from port_queue and port_wait

This argument is not used and always must be one. I tried implementing
some reasonable behavior for this parameter on port_queue, but it wasn't
clear what to do when we encounter an error during the queue operation
because there isn't an out parameter for how many we succeeded in
queuing.

Rather than having a half-baked feature, we should just remove the
parameter and add another function that does vectorized versions of
these operations if we need that in the future.

Change-Id: Ifffcd6d705442cc830912a67c8117ffe5ec8295d

[zx] Call zx_port_{wait,queue} with correct count

In the near future zx_port_{wait,queue} will not accept a count of zero.
Change all zircon callers to call these functions with a count of one.

ZX-1291 #comment

Change-Id: I5697208c7931a5e117f469c901c78f65615eeee0

[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

[kernel][x86] Enable UMIP if present

UMIP prevents usermode from executing SGDT, SIDT, SLDT, SMSW, and STR.
In our system, there isn't a need for a usermode process to invoke
these.

ZX-1530 #done

Change-Id: I7047c63c9c7a2d02325eb733e4e2d94dd2d3a538