os/fs: Use module-local uv_loop_t instance

This event loop is just a stub instance used in synchronous libuv function
calls, it needs to be decoupled from the main event loop in order to run it from
another thread.

src/nvim/main.c

@@ -138,6 +138,7 @@ static const char *err_extra_cmd =
 /// Needed for unit tests. Must be called after `time_init()`.
 void early_init(void)
 {
+  fs_init();
   handle_init();
 
   (void)mb_init();      // init mb_bytelen_tab[] to ones