Add a debugging sysctl, debug.vm_lowmem, that when assigned a value of 1 will trigger a pass through the VM's low-memory handlers, such as protocol and UMA drain routines.  This makes it easier to exercise these otherwise rarely-invoked code paths.
Add a debugging sysctl, debug.vm_lowmem, that when assigned a value of
1 will trigger a pass through the VM's low-memory handlers, such as
protocol and UMA drain routines.  This makes it easier to exercise
these otherwise rarely-invoked code paths.

MFC after:	3 days

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