input: stream_set_blocking(): libuv impl

- Create a private libuv loop instead of re-using uv_default_loop(), to
  avoid conflict[1] with existing watcher(s) on the fd.
- Expose the global "input" fd as a getter instead of a mutable global.

[1] .deps/build/src/libuv/src/unix/core.c:833:
      uv__io_stop: Assertion `loop->watchers[w->fd] == w' failed.

src/nvim/os_unix.c

@@ -181,6 +181,7 @@ void mch_exit(int r)
   ml_close_all(TRUE);           /* remove all memfiles */
 
   event_teardown();
+  stream_set_blocking(input_global_fd(), true);  // normalize stream (#2598)
 
 #ifdef EXITFREE
   free_all_mem();