[zircon][syscalls] Make zx_time_t and zx_duration_t signed types

This change is part of a hard transition and breaks the layers above.

zx_time_t and zx_duration_t are now int64_t. ZX_TIME_INFINITE is now
INT64_MAX.

A negative time value passed to a syscall is treated as any other time
value that's "in the past".

Test: CQ; new unit tests; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I2a020cb2cf3dd7212608e24c1ee90cec58dd51ef

[docs][syscalls] Document more syscall right requirements

ZX-2399 #done

Test: no functional change
Change-Id: Ib1dd0ff19eac9e7ceb6227e83817eeb1d0d1dbdc

[docs][syscalls] Add TODOs about documenting system call rights

ZX-2399 #comment

Test: no functional change
Change-Id: I1f285c677e2444fcf1d720d7ecbd9f490c303962

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[kernel][magenta] Add slack options to timers

The long requested feature now exposed to user mode. Now
the timer slack and be early, center or late.

The default behavior of 0 is "center" which was the existing
behavior so there no effective change on the existing
build.

Tests included with the caveat that better tests
exist already via "k timer_tests".

Change-Id: If1787e30eda4c06be003dea715e92c3f240841ca

[kernel][magenta] start timer_start deprecation

In the future, only one-shot to be supported.

- Provides timer_set
- Keeps timer_start
- Tests for both

Also removes MX_TIME_MIN_DEADLINE: now zero
can be used to fire the timer now.

Change-Id: I0894755c37b752e11bb1938dd35aca7cef2edebe