testbench: add kill_thread()

And use it in xjoin_thread() when it fails to join the given thread within the
time given.
test: thread: add tests on thread halt on missing pager or exception handler
exception: only store up to UntypedWords given in exception reply message

Short replies won't store the later registers. This is appropriate.

(also, don't printf() when the pager cannot be found. the previous changeset's
test covers for this now.)
testbench: add the IPC_FAIL() macro

That took long enough.
test: ipc: add test for the C bit in typed transfers

Fails in one test point, which is nice.
ipc: set stringitems' C bit correctly in the receiver

This adds the `tag' field to ipc_state. It must be retained because a pager
interaction might stomp the sender's saved MR0. {citation needed}