[public][ulib][dev] Add __FALLTHROUGH macro for warning suppression

GCC 7 enables -Wimplicit-fallthrough with -Wextra, so we'll get it.
Clang has it available, so start using it and clean up code accordingly.

The GCC feature will actually recognize some formulaic comments to
suppress the warning. But there's also an explicit syntax, and
C++17 added a standard one. So use that instead, via the new macro
__FALLTHROUGH in <zircon/compiler.h>.

TC-36 #comment Clean up code in preparation for more -Wimplicit-fallthrough

Change-Id: I28309c91c32db1313d52f4255836c5ea687e141f

[kernel] Use zx_ticks_t as ticks unit, rather than uint64

Change-Id: I189a45336a9c0544de6a93a50f83d9b088dec77e

[ping] Implementation of ping

Change-Id: I993d50822119ee38254c5470e2912311c8b99a0b