[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[time] Switch callers of syscalls with timeouts to use conversion function

This is in preparation for moving syscalls to absolute deadlines rather
than relative timeouts.

Change-Id: Ic85abbf76f34fbce7899c2505be840e3d3dcf7ff

[ulib][uapp] Use atomic types with C11 atomic operations

C11 atomics are only defined on atomic types. C11 atomics implementation
in GCC doesn't seem to be enforcing this property, but the one in Clang
does, and it causes compilation errors.

There are three different ways to specify C11 types:

_Atomic const int * p1;
const _Atomic(int) * p3;
const atomic_int * p2;

While the first two are C11 specific, only the third is compatible with
C++11 atomics; the equivalent types are declared within the std
namespace in the <atomic> header. Since many of Magenta's headers are
used in both C and C++ code, this is the prefer option.

Change-Id: I52d6734c1149e9a28059d786e8645e5a0189ac30

[time] Update some code to use new time convenience macros

Change-Id: I6289116690e46948c0f2d39e68939331173335d7

[utest][pthread] Test pthread_key_t destructors

After the recent thread refactorings, pthread tsd destructors work.

Change-Id: I8d95758c7273b8875445b60dbcb7c64cbf244007

[system] align license and license banners with Fuchsia

This adjusts the Magenta userspace code to match the prevailing
license in Fuchsia (three-clause BSD), using the same banner
and license text.

Change-Id: I6b6d79435c5dbaa32027888874849e4f47aaa7be

[utest] Support categories

This will produce single binary from mxr tests and also seperate binary
for each test

Change-Id: I9345fbeab5751dda356cfd47a766c09957211923