[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

[syslog] Fallback when socket closes on other end.

Change-Id: Idcd34d023d5c442ebcfdd455dc6bc7a3b336b3fa

[syslog] Add fallback mode support.

This mode will be used when logger fidl service crashes or fails to
process messages. This mode can also be used for specific errors with
socket writes.

Change-Id: I02ab21725d590a1f9632a153ca9d00275057913c

[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

[syslog] No longer attempt to format raw messages.

FX_LOG() and fx_log() no longer attempt to format message strings.

Do not pass those message through printf as they are preprocessed.

Change-Id: Ie9b784859ca64fe31d2ab6e1747bf83f31d0016e

[syslog] Remove version from fx_log_metadata

Also make dropped_logs field 4 bytes long to align struct properly.

Change-Id: I94e272343ce96f27383952f2a7941d547c2f1ade

[syslog] Format code

Formatting using //script/git-file-format

Change-Id: I6f572735d2ee510ebf0348db34916656974f365c

[syslog] Add support for writing to socket

ZX-1665

Change-Id: Ie21550b6784deeebc4ed8b752534108dfcbd1640

[syslog] Fix errors to make it work with garnet

Change-Id: I946f2ec63358a2e1921a91606df8c45aba53d8dd

[syslog] Initial implementation

This implementation supports only supports writing to file descriptors
and defaults to stdout.

Change-Id: I2aaca1cb98e0f8951e1b791c40250dc503a93b3f