Keeping it enabled leads to nxagent consuming 100% of CPU resources
after some time.

Older code used three different timers for each DPMS mode (standby,
suspend, off), with each timer checking that the elapsed idle time is
strictly less than the DPMS mode timeout value.

Newer code started merging DPMS and ScreenSaver timers into a single
one, with checking DPMS modes and timeouts in a fall-through fashion.
The code expects that, if a timeout is reached, the mode is set
accordingly, so that the old timeout is disregarded next time.

Since we stub out DPMSSetMode() in nxagent, this doesn't happen.

In this case, the old DPMS timeout will be checked the next time around
and we will be calculating DPMS_MODE_TIMEOUT - IDLE_TIME. If IDLE_TIME
is bigger than DPMS_MODE_TIMEOUT, we run into problems with unsigned
integers. What happens next will be *another* overflow once DoTimers()
is executed and from that function SetTimers(), which will add the
current timestamp to the timeout value. Later functions subtract the
current timestamp again (which will be even higher since some time
elapsed) and underflow the value again.

It looks like this leads to consistent firing of the timer - something
we do not want to have.

For new, disable DPMS support in nxagent. We do not execute it on actual
hardware that could change DPMS modes.

At a later time, implementing a DPMS timeout/mode passthrough to the
connected X server/display might be beneficial.

Fixes: ArcticaProject/nx-libs#671