Fix a bug in the debug patch.

Here is a small patch that adds a few KASSERTS. There might be some value in seeing which of them is tripped first on the way to this failure.

It seems that this change broke something:
Hi Doug,

Restore the invariant-checking code that fell out of the last patch.

Reformat vm_map_entry_{pred,succ}.

Would there be a problem with changing the #defines:
#define PQ_NONE 255
#define PQ_UNSWAPPABLE 0
#define PQ_LAUNDRY 1
#define PQ_INACTIVE 2
#define PQ_ACTIVE 3
#define PQ_COUNT 4

VME_ -> VM_MAP_ENTRY_
Remove purposeless counter.
Move KASSERT out of loop.

Combine two loops in ptrace_vm_entry into one, so that "entry->next" doesn't have to appear.

Correct an off-by-one error.

Update after r352433 committed changes to vm_map_entry_unlink that were part of this change.

In D20664#472328, @alc wrote:

How does this perform on the microbenchmarks that you've previously used?

Address reviewer comments. Replace another stray 'precessor' with 'predecessor'.

Rewrite comments describing vm_map_lookup_helper.

Discard parts that overlap/conflict with changes applied since this change was proposed.
Extend this change to completely eliminate the 'prev' field from vm_map_entry.

All existing callers guarantee that the page does not have a pre-existing dequeue pending.

Can you add a KASSERT to express that in the callee?

Apply reviewer directives.

Seems plausible.

Simplify the patch.

Apply reviewer suggestions.

Ready for review....

.Dq late

line wrap.

Apply alc comments.

Use howmany in one more place.

Apply reviewer suggestions.

Make associated manpage fixes, including a clumsy attempt to address a savecore issue.

In D20863#459908, @dougm wrote:

Without changing the behavior of this patch, add a SW_TRIM flag and a priority field to swdevt. Pass those from sys_swapon to kern_swapon (which does the work formerly done by sys_swapon) to swaponsomething, which creates the swap object.

Without changing the behavior of this patch, add a SW_TRIM flag and a priority field to swdevt. Pass those from sys_swapon to kern_swapon (which does the work formerly done by sys_swapon) to swaponsomething, which creates the swap object.

Update log messages to distinguish cases where trimming is aborted before it gets started. Currently, a stretch of log entries like:

Fix function naming mismatches in KASSERTS. In wire/unwire, make sure to clip after first entry found in transition.

Don't unlock on early exit from map_wire_locked.

Get rid of the remaining uses of vm_map_simplify_entry.

Update after checking in overlapping changes.

Tidy up comments.

Apply reviewer suggestions. Make another style(9) fix.

Use vm_map_try_merge_entries instead of vm_simplify_entry in vm_map_inherit.

In looking for the map entry that includes the page before page 0, use &map->header explicitly instead of using vm_map_lookup_entry.

In one comment, connection -> descriptor.

Consume reviewer suggestions.

Rename vm_map_merge_entries to vm_map_try_merge_entries.

Fix broken log entry.