lib: copy native words in memcpy_forward()

So on a 64-bit platform this'll do 8 bytes per iteration. The in-kernel
memcpy() could likely wrap around the one in the library and do SSE things
(with the attendant coprocessor switch) when the buffer is sufficiently large,
or when the kernel context has already been loaded.
ipc: resolve partner in ipc_send_half() caller

This removes the ipc() function, which had exactly one call site. Part of a
series to simplify sys_ipc() etc. in anticipation of partner scheduling.
test: ipc: check that the kth range cannot be ipc'd to

Doesn't pass.
ipc: forbid user IPC to system range

Except for messages from sigma0 to its pager, and the in-kernel self-test
messages from roottask space to the same (only when enabled), IPC to the
system range now fails with "partner not found in send phase".
ipc: restore sigma0 printf

I.e. permit IPC FromSpec in the system range, but only for s0_pager and only
from sigma0's space.
test: ipc: Ipc must reject FromSpec in system range

And it does.

As it happens, this is the 1200th changeset in mung's git tree. Not that I'm
superstitious or anything.