Extract mount specific code to new file

As fusefs.rs has grown larger and larger, it's nice to split some code
out.

Bug: 205750213
Test: m

Change-Id: Icc51642dd8c3ebf466e367e22f88dd9221c3e3b5
diff --git a/authfs/src/main.rs b/authfs/src/main.rs
index 00a4614..a083381 100644
--- a/authfs/src/main.rs
+++ b/authfs/src/main.rs
@@ -311,7 +311,7 @@
     let mut authfs = AuthFs::new(RemoteFsStatsReader::new(service.clone()));
     prepare_root_dir_entries(service, &mut authfs, &args)?;
 
-    fusefs::loop_forever(authfs, &args.mount_point, &args.extra_options)?;
+    fusefs::mount_and_enter_message_loop(authfs, &args.mount_point, &args.extra_options)?;
     bail!("Unexpected exit after the handler loop")
 }