[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[build][musl] refer to musl libc as ulib/c

This makes the build rules and the intermediate
output files easier to understand, without having
to modify the path where musl libc lives.

The biggest observeable difference is the build
intermediate files:

$ ls build-magenta-pc-x86-64/ulib/c
config-module.h libc.abi.h libc.abi.stamp libc.so libc.so.id libc.so.strip
libc.a libc.abi.o libc.a.opts libc.so.abi libc.so.opts third_party

which previously were at build-magenta-pc-x86/ulib/musl
and named libmusl.*

and the replacing of ulib/musl with ulib/c in the library
dependencies of all the rules.mk files (the vast bulk of
this change)

Change-Id: If9c5d522099d2c9ac5bdb666b8e67a2eb3d316ee

[magenta] Migrate Magenta's uses of mxio_get_startup_handle

Leave the compatibility shim in place until Fuchsia-at-large migrates.

For MG-228: "get startup handles should not live in mxio"

Change-Id: I5bf53388585520bf7d4b5acc31902bfc7797489d

[utest][exception] re-enable test

Change-Id: I375675e92a76f420eb6304a4421f540891bacef3

[utest] Make exception test fully build, but as a uapp

In the prior commit disabling it, I wasn't aware of the ability to just
move it to /boot/bin rather than /boot/test by manipulating MODULE_TYPE.

Change-Id: I73bd0b0906f522ac6fa55841e144bc77fac8e5d7

[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

[system][ulib][test-utils] Create test-utils library.

test-utils is moved from unittest to its own library.
The reason for this is that test-utils uses mxio but
unittest cannot use mxio. See system/ulib/unittest/README.md
and system/utest/core/README.md.

Change-Id: Id79ea8a4cf8cc948994cef5bcc3dfe035e90bff4

[system][utest][exception] Rename exceptions to exception.

Most tests use the singular (io-port, message-pipe, etc.)
and I'm finding my muscle-memory when typing prefers the singular.

Change-Id: I053c77f9bfa9353e0d20956040c2db70f65b98f9