[zircon] non-resizable VMOS

This CL introduces the first option for VMO creation
ZX_VMO_NOT_RESIZABLE that 'locks' the size of a root (non-clone) VMO

Also enables the same for COW clones via ZX_VMO_CLONE_NON_RESIZEABLE

This is meant to answer safety hazards of having a client pass
VMOs to servers which the client can resize while the server is
copying data into via a memory map.

Today this is done via ZX_INFO_HANDLE_COUNT but that turns out
is racy.

This CL also cleans up vmo-tests so they using proble_for_read
and probe_for_write instead of using unrelated syscalls to test
memory mappings.

TEST=added unittests in vmo_tests

ZX-2279 #comment progress

Change-Id: If69ce772dcccd92db83f5bda313ae2343ebb448e

[kernel] Move unittest.h to include/lib/unittest/

Move unittest.h deeper in the tree so include statements will say:

Change-Id: I85cb027a53530cd2003d813f004aaea3509f9f40

[kernel] Add unit test for MessagePacket

Move UserMemory test helper out of mbuf_tests.cpp so it can be reused.

ZX-1881 #done

Change-Id: I89a3683f284ce26046ea49b9010e6173c709fbc1