[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[cksum] modernize crc/adler code

- use modern standard types
- use ansi C definitions
- regularize type usage
- make buildable in kernel again

Change-Id: I16ba18f8c75dcbc3b9e1403bcb0c7fcfb97b1af0

[system] Move system/* includes to magenta/* under global.

Change-Id: I9b8673a16162da3bd1328a2eae3fbd855f1a7730

[lib][cksum] new style build config of shared user/kernel libraries

1. The kernel build globs all includes into one global -I space, but
the user build depends on public includes being relative to the
module source, so the "canonical" source needs to live in the
.../ulib/$module/... space so .../ulib/$module/include/...
contains the public headers:

This is third_party/ulib/cksum for libcksum

2. The kernel version of the module simply needs to reference the
canonical source and contains only a rules.mk, as in:

third_party/lib/cksum

3. If there are public headers, the kernel side rules.mk must
add them to the kernel include space, like the above does:

KERNEL_INCLUDES += $(SRC_DIR)/include

Change-Id: Ia1a5af22844f22e2c027dcaea7e7b7cf35d00ab2