Downgrade errors about incompatible service config to warnings

If service type is changed from socket to executable, some config
options are not applicable anymore. Do not fail execution but just log a
warning. This is also relevant for migration in the other direction - if
user has an executable service that is updated to a socket service, user
may want preserve the executable variant (which is also done by the
package manager if user has modified said service) - in which case, the
config for socket variant should not prevent this configuration from
working.

This has been reported to happen with qubes.UpdatesProxy service.

Fixes QubesOS/qubes-issues#9299

libqrexec/exec.c

@@ -748,21 +748,19 @@ int find_qrexec_service(
     if (euidaccess(path_buffer.data, X_OK) == 0) {
         /* Executable-based service. */
         if (!cmd->send_service_descriptor) {
-            LOG(ERROR, "Refusing to execute executable service %s with skip-service-descriptor=true",
+            LOG(WARNING, "Warning: ignoring skip-service-descriptor=true "
+                         "for execute executable service %s",
                 path_buffer.data);
-            return -2;
         }
         if (cmd->exit_on_stdout_eof) {
-            LOG(ERROR, "Refusing to execute executable service %s with "
-                       "exit-on-service-eof=true",
+            LOG(WARNING, "Warning: ignoring exit-on-service-eof=true "
+                         "for executable service %s",
                 path_buffer.data);
-            return -2;
         }
         if (cmd->exit_on_stdin_eof) {
-            LOG(ERROR, "Refusing to execute executable service %s with "
-                       "exit-on-client-eof=true",
+            LOG(WARNING, "Warning: ignoring exit-on-client-eof=true "
+                         "for executable service %s",
                 path_buffer.data);
-            return -2;
         }
         return 0;
     }