new i386/amd64 machdep.lidsuspend sysctl which decides whether a lid

new i386/amd64 machdep.lidsuspend sysctl which decides whether a lid
close causes a suspend.  resumes are not tied to this; they happen
unconditionally.  this is a temporary knob for the 4.8 release; afterwards
it will move to a more convenient place
much moaning discussing where to put the knob with kettenis
tested by various including phessler