Use the correct null constants for Mach threads, tasks, and hosts

Use the correct null constants for Mach threads, tasks, and hosts.

This uses THREAD_NULL, TASK_NULL, and HOST_NULL in preference to
MACH_PORT_NULL and kMachPortNull. These constants are correctly-typed
(thread_t, task_t, and host_t) and result in more readable source code,
especially where thread and task parameters appear together as they do
in exc_*_variants.

TEST=util_test
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/8c7872e9e0218ba2a86f546826e8be78521abfed

[Mark Mentovai, 2014-10-10 18:00:12]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2014-10-13 15:03:01]

LGTM

[Mark Mentovai, 2014-10-13 16:59:25]

Committed patchset #1 (id:1) manually as
8c7872e9e0218ba2a86f546826e8be78521abfed (presubmit successful).