[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[tests][thread-initial-state] change the XMM test to avoid using ptest

This makes it work on machines without SSE4.1.

Change-Id: Iee3f1970c238d00043fbfc0f3a104e01c5630a15

[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

[uapp] [utest] Switch most uapp and utest builds to dynamic linking

Nearly every uapp and utest that can be dynamically linked is now.
The mxr-tls test is omitted because it's broken when dynamically
linked; that will probably be moot after the forthcoming TLS revamp.

A few test helper programs cannot use dynamic linking as written
because they get run with a nonstandard bootstrap message that does
not include the magic required for the dynamic linker's startup.

I didn't touch anything using ulib/ddk because I didn't bother to
figure out how that fits together.

Change-Id: I93b08960d946e23cd4ac89216e6d32789f99405a

[x86][uspace entry] Ensure all register state is zeroed on usermode entry

This cleans up some comments in uspace_entry that are now stale, and
adds support to the thread-initial-state test for inspecting the YMM
registers.

Change-Id: I799abca0fa95d0c146cf735e6752fc54dbc76e60

[syscalls][api] rename _magenta_*() to mx_*()

The _magenta_ prefix is pretty unwieldy.
This change simplifies that a bit.

This was done mechanically, and boots, runs, and passes tests
on qemu arm64, amd64, and NUC.

Change-Id: I8eb07db6ed6f4b8cac96344cc4ad65ce3f85f917

[system] final round of underscore removal

Change-Id: I4b2f5b4a8fd0d6dcf25e0f2db913313569a15463