Detect the rig's binaries, and take the machine out of its comments

The comments described the machine the rig was written on -- "this
machine has no display", "there is a real GPU here", an Android
emulator's GLX quirk -- which says nothing to anyone reading it from a
different checkout. What the reader needs is what the script supplies
and why, which is now all they get.

`sway`, `swaymsg` and, when `--shot` is passed, `grim` are checked up
front and named in the failure, rather than surfacing as a compositor
that would not start.

The `# shellcheck disable=SC2086 -- prose` directive did not parse, so
the suppression was not in effect; the prose moves to its own line.
Clean under shellcheck now.

scripts/headless.conf

@@ -1,13 +1,13 @@
-# The compositor `scripts/run-headless.sh` starts, because this machine has no
-# display. Nothing here is meant to be looked at directly; `grim` is.
+# The compositor `scripts/run-headless.sh` starts, so that an example has a
+# surface where there is no display. Nothing here is meant to be looked at
+# directly; `grim` is.
 #
-# No Xwayland: winit talks Wayland natively, and starting an X server is a
-# second thing to go wrong for no gain. (`emu`'s config forces it because the
-# Android emulator's renderer speaks GLX.)
+# No Xwayland: winit talks Wayland natively, so an X server is a second thing
+# to go wrong for no gain.
 xwayland disable
 
-# A desktop-shaped output, since this is the desktop half of the port. Larger
-# than the window an example opens, so nothing is scaled or clipped.
+# The default output, overridden per run by `--mode`. Larger than the window
+# an example opens, so nothing is scaled or clipped.
 output HEADLESS-1 mode 1920x1200@60Hz
 
 default_border none