drm: put the display wake behind its own compile gate and a runtime option

everything else in this backend READS: it captures a scanout. the wake
WRITES, injecting one synthetic pointer event from the root service
into the user's session. that is a different kind of operation and it
should be switchable on its own, at both levels.

- compile: a `drm-wake` feature on top of `drm`. every wake-only item
  is gated and drm_enumerate_settled has two definitions, so
  `--features drm` builds the same capture path with no wake code in
  the binary. verified on a RELEASE artifact with both controls: the
  drm markers are present (Started drm ipc server) and the wake string
  is gone. the unattended deb passes drm-wake, so answering an
  objection is one word in build.py rather than a revert.
- runtime: `enable-drm-display-wake`, server-side, the same shape
  rustdesk already uses for the closest thing it does to this
  (keep-awake-during-incoming-sessions, which PREVENTS sleep where
  this RECOVERS from it, and is acquired only once a connection
  exists, which is too late for a host that cannot be reached).
  the `enable-` prefix is load-bearing: option2bool reads an absent
  value as ON, and a host whose screen went dark is the case the
  unattended package exists for. set it to "N" and the service stays
  read-only with respect to input.

the key is declared in this file rather than in hbb_common's `keys`
module, where rustdesk's own option constants live: hbb_common is a
submodule of a repo we do not control, so a constant there could only
land after an upstream change plus a submodule bump. the option system
reads by string, so registration is not required; the cost is that the
key is set in the config file rather than the settings UI, which is
how an unattended host is configured anyway.

build.py

@@ -298,6 +298,13 @@ def get_features(args):
         if windows or osx:
             raise Exception('--drm is Linux only')
         features.append('drm')
+        # The display wake is its own compile gate on top of `drm`, and the unattended package is
+        # exactly where it belongs: that variant exists to reach a machine nobody is sitting at,
+        # and a machine whose screen went dark is the case it is for. Dropping `drm-wake` from
+        # this line builds the same capture backend with no wake code in the binary at all --
+        # which is the point of having the gate, so the answer to an objection is one word, not a
+        # revert. It is ALSO switchable at runtime; see OPTION_ENABLE_DRM_DISPLAY_WAKE.
+        features.append('drm-wake')
     if osx:
         if args.screencapturekit:
             features.append('screencapturekit')