[zircon][kernel] Use proper time types and overflow-safe arithmetic

This change is preparation for converting zx_time_t and zx_duration_t
from unsigned to signed types (ZX-2100).

1. Fix several places where the wrong time type was used (e.g.
zx_time_t instead of zx_duration_t). Because these are simple typedefs
it's easy to use the wrong one. While it generally doesn't cause a
problem, using the wrong type can be misleading and lead to bugs like
passing a relative time when an absolute time is expected.

2. Update kernel/ and some other parts of zircon to use the new
overflow-safe arithmetic functions from zircon/time.h.

Performance: Ran "/pkgfs/packages/zircon_benchmarks/0/bin/app -p" on
NUC before and after this change. Differences were withing the noise,
some slightly faster, some slightly slower.

Test: CQ

ZX-2100 #comment cleanup before conversion

Change-Id: I75d074e5850ade5385decd9bbf330497bf7610e9

[clock] zx_clock_get(ZX_CLOCK_MONOTONIC) -> zx_clock_get_monotonic()

Test: no functional change

Change-Id: Iaa513c521d96b178458acc707be3646067971701

[syscalls] Rename zx_time_get to zx_clock_get

This change frees up the get() method in for a zx_time_t accessor in the
zx::time class.

Change-Id: Ia4cdc7e3e558df5ba0cfeedb27a442abf4d6f305

[zx] Rename libmx test to libxz, mxr-mutex test to zxr-mutex

The Lesser Renaming is here!

Change-Id: I67a24e0ac9baa888e3fd08c97a22d76d17e3bf56