Make it possible to free overlapping regions (effectively allowing
freeing parts of regions that are already free) when the EX_CONFLICTOK
flag is passed to extent_free.

ok jmatthew@

Make sure we do not loop when allocating an extent region with EX_CONFLICTOK
and the whole extent is used; the current code computations would wrap.
Found the hard way by jsg@, fix discuss with kette

Make sure we do not loop when allocating an extent region with EX_CONFLICTOK
and the whole extent is used; the current code computations would wrap.
Found the hard way by jsg@, fix discuss with kettenis@, and you get a
regress test for free (which will spin if you compile it again an old
subr_extent.c)

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Add regression test for overlapping allocation that should coallesce but
currently doesn't.

Add EX_CONFLICTOK flag that allows an allocation to conflict with existing
allocations, making sure that the union of all space is allocated.

ok deraadt@

Add a regression test for the extent subroutines; from NetBSD.