Use xochitl instead of remarkable-shutdown in rm2fb-server

In the last software relase (2.6), new waveform modes were added to
xochitl to make the pinch-to-zoom feature work. Unfortunately, the
remarkable-shutdown binary was not updated to support those new
waveforms. As a consequence, using pinch-to-zoom with rm2fb results in a
less smooth experience than without rm2fb, since the screen gets
refreshed less frequently during the gesture.

See:

* <#322 (comment)>
* <ddvk/remarkable2-framebuffer#56>

To address this issue, this PR changes the rm2fb-server to preload the
`/usr/bin/xochitl` binary instead of `/usr/bin/remarkable-shutdown` in
both the `rm2fb-server` command and `rm2fb.service`.

A start condition is added to `rm2fb.service` to only start if the
librm2fb_server library actually exists on the filesystem. This is to
avoid having two concurrent xochitl processes running, as the loader
ignores non-existent files provided in LD_PRELOAD.

Test plan:

* Manually run `rm2fb-server` and `rm2fb-client xochitl`, make use of
  the pinch-to-zoom gesture, and make sure no message of the form
  `Unable to complete update: invalid waveform ( 6 )` appears in the
  server process.
* Check that the display continuously updates when using the
  pinch-to-zoom gesture, even if the `rm2fb.service` service is running
  and xochitl is started with `rm2fb-client xochitl`.
* Remove the `/opt/lib/librm2fb_server.so.1` symlink and restart
  `rm2fb.service`. The service should not start. Make sure that there’s
  no duplicate `xochitl` process in this case.

package/rm2fb/package

@@ -8,7 +8,7 @@ maintainer="raisjn <[email protected]>"
 license=MIT
 pkgdesc="Interface to the reMarkable 2 framebuffer"
 url="https://github.com/ddvk/remarkable2-framebuffer"
-pkgver=1.0.1-1
+pkgver=1.0.1-2
 section="devel"
 
 image=qt:v1.1

package/rm2fb/rm2fb-server

@@ -1,2 +1,2 @@
 #!/usr/bin/env bash
-LD_PRELOAD=/opt/lib/librm2fb_server.so.1 exec "$(command -v remarkable-shutdown)"
+LD_PRELOAD=/opt/lib/librm2fb_server.so.1 exec "$(command -v xochitl)"

package/rm2fb/rm2fb.service

@@ -8,9 +8,10 @@ After=opt.mount
 StartLimitInterval=30
 StartLimitBurst=5
 Conflicts=
+ConditionFileNotEmpty=/opt/lib/librm2fb_server.so.1
 
 [Service]
-ExecStart=/usr/bin/remarkable-shutdown
+ExecStart=/usr/bin/xochitl
 Restart=on-failure
 RestartSec=5
 Environment="HOME=/home/root"