[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[bootfs] streamline the bootfs format

- provide common header and entry structs in bootdata.h
along with documentation comments
- stop treating bootdata headers as part of bootfs
- introduce a simple header with magic and directory size
(allows easy mmap'ing of just the directory)
- align bootdata directory entries on U32 boundaries
- adjust mxio and userboot to deal with the changes

Change-Id: I09f96088f939dd541a6b23d70fcd8606d1af4f28

[core][userboot] Provide a basic loader service in userboot

This makes it possible for the initial program (usually devmgr)
to load additional shared libraries.

Change-Id: Ief65d8ec8705d47bbacbbeb85ec28811d4c3725e

[core][userboot] Use mx_vmo_clone for bootfs files in userboot

Avoid the data copying and extra memory for the bootfs files
that userboot looks up.

Change-Id: I592106c1db5c1dc38739c696f2762ca65bf24925

[vmar][unmap] Remove size=0 compatibility hack

For compatibility with the old interface, we were allowing size=0
to be passed in to mean "unmap the entire mapping that contains
the base address".

Change-Id: I9f6f560a685aff37fcd56d92ffca4ed94655b88b

[vmar][userboot] Convert userboot to using VMARs

userboot now uses the new VMAR syscalls for mapping, and passes
its child the child's root VMAR handle

Change-Id: I3678db196e2ab737e9725af6fbf1e8a75ce48070

[organization] move essential userspace components to system/core/...

userboot, devmgr, acpisvc, crashlogger, netsvc are essential to the
system booting and running. Pretty much everything else in system/uapp/..
is nonessential.

Change-Id: Ibbcf2b1c388bdefc28699226eab02e90aab895f0