[socket] Modify the error reported when writing a very large datagram packet

When attempting to write a datagram packet larger than the socket's
entire capacity, we previously returned ZX_ERR_SHOULD_WAIT. This is
misleading: the wait can never be satisfied, as socket capacity cannot
grow. This CL changes it to report ZX_ERR_OUT_OF_BOUNDS instead.

ZX-2553 #done

Test: modified /boot/test/core/socket accordingly, and added `k ut
mbuf` tests

Change-Id: I40ac9e8ec0c1e228ee19219499e951e7e6870a8a

[kernel] Don't crash when reading from empty MBufChain

ZX-1847 #done

Change-Id: I4af3b7ef72cf7d2608200561f12990f4310698ab

[kernel] Disallow zero-length datagram in MBufChain

ZX-1848 #done

Change-Id: I3f34119e0a805b99eb96c8a2cabe25b81d0bea4a

[kernel] Run unittests with user address space

Update mbuf_tests.cpp to use user memory insetad of fake_user_ptr.

Remove fake_user_ptr and testable_user_ptr.

Convert unittest.c to C++.

Tested this change by running kernel unittests via "k ut all" and
kernel.shell

ZX-1914 #done

Change-Id: I9898cb23da109485dee6db06de11cbd0928a6ba9

[kernel] Add unit tests and comments for MBufChain

Add unit tests for MBufChain that make use of testable_user_ptr and
fake_user_ptr.

Add comments to MBufChain.

ZX-1846 #comment initial tests
ZX-1852 #comment prereq

Change-Id: I6626b5e05e56611006da3de0f8d7deae482fb474

[user_ptr] Enforce IN/OUT/INOUT semantics

Change-Id: I54e92ed999c1965114aa80aeffe8d0fc16184b40

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be

[lib/magenta] Rename kernel/lib/magenta to kernel/object

TODO: Rename magenta.{cpp,h} and other instances of "magenta" in this
module.

For MG-1091 "Rename kernel/lib/magenta"

Change-Id: I2abc316f543798e00d3a4d1c2c83195da26e6836