[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 testable_user_ptr and fake_user_ptr

Add testable_user_ptr and fake_user_ptr to facilitate testing kernel
code that deal with user_ptr's.

ZX-1857 #done

Change-Id: If3e516711034cff1d94b16bdc98ae082ef149600

[user_ptr] Remove is_user_address and weak arch_copy_to/from_user

The only callers of user_ptr::is_user_address don't actually need it.

There's also no reason to keep the weak memcpy()ing versions of those
functions around.

Change-Id: I2122c6affc84b0f6557d8a50da7667d454a17a0e

[magenta] Initial commit