acme: add threadmaybackground

Without threadmaybackground acme will remain a white window when
started from a hotkey daemon such as skhd like this:

/usr/local/plan9/bin/9.rc acme -c3

src/cmd/acme/acme.c

@@ -57,6 +57,12 @@ derror(Display *d, char *errorstr)
 	error(errorstr);
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {