Implement the guts for "suspend-to-idle" on amd64. This enables suspend

Implement the guts for "suspend-to-idle" on amd64.  This enables suspend
on machines that don't support S3.  In its current state it doesn't save
a lot of power, but this should improve over time.  Implementation of
wakeup methods is incomplete which means that some machine can't resume
at the moment.

ok mglocker@, mlarkin@, stsp@, deraadt@