ipc: fix assert crash for string transfers ending at segment boundary

The tests for interspace transfers still fail, so this changeset fixes panics
only.
test: string: add full_string_buffers to provoke typed IPC bugs

The test has iter-flags for whether the receive is forked or threaded and
whether IPC will happen as active send or active receive. The combination of
forking and active receive provokes an error from the memcpy_from() fastpath
in ipc_typed.c .
ipc: disable memcpy_from() fastpath in interspace string transfer

Known not to work. Will be fixed.