testbench: fix thread restart bug, allow local TIDs to join_thread()

The former was introduced with the LTID fix, earlier, and the latter is
necessary to not require test code to hold the library's hand.
add L4_SameThreads() to <l4/thread.h>
sched: switch space, cop state in switch_thread_u2u()

Oops.
clean up debugs in sched, exception

The latter's became more useful, and the former's hadn't been anything but
noise for a long time.
ipc: fix propagation, sender lookup, and LTID returns

tid_return() was added to thread.c to cough up local TIDs when @self == @t,
and global TIDs otherwise.

This also changes ipc_send_half() to use propagation to matching for active
send. That's so far untested though.
test: ipc: add ipc/api/propagation

This doesn't cover propagation to a receiver that's expecting a reply from a
different thread, i.e. in active send with non-wildcard match.