cli: add command to restart the compositor in place

wire/jay_compositor.txt

@@ -105,6 +105,10 @@ request get_color_management (since = 14) {
     id: id(jay_color_management),
 }
 
+request reexec (since = 17) {
+    id: id(jay_reexec),
+}
+
 # events
 
 event client_id {

wire/jay_reexec.txt

@@ -0,0 +1,11 @@
+request arg {
+    arg: str,
+}
+
+request exec {
+    path: str,
+}
+
+event failed {
+    msg: str,
+}