unmap kernel memory immediately

This makes kmem_cache_find() extra illegal for objects that were allocated in
subsequently destroyed caches. So much so as to immediately panic the kernel.
So the corresponding test was removed from t_slab.c, and life goes on.
heap: remove FIXMEs

Two were demoted for not being show-stoppers, and one removed.
mapdb: move int_eq() into <ukernel/util.h>

And remove a copy from testbench's tsd.c .
space: unmap old KIP in space_set_kip_area()

This is a rare code path that's so hard to test that it'll just be assumed
correct.

The sole use case would have an OS personality mapping a "syscall glue" page
in somewhere, which'd be rewritten in lockstep with the relocated KIP;
presumably all threads in the space would be stopped using ExchangeRegisters,
however, any thread that was in a system call when the KIP moves will return
into non-KIP code (or just a hole in the address space). This could be avoided
by swapping out such threads' EIP bits to refer to the new KIP address, but
again, too hard to test so let's punt.