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/misc1.c

@@ -2679,6 +2679,7 @@ void preserve_exit(void)
 
   // Prevent repeated calls into this method.
   if (really_exiting) {
+    stream_set_blocking(input_global_fd(), true);  //normalize stream (#2598)
     exit(2);
   }
 
@@ -2699,7 +2700,6 @@ void preserve_exit(void)
   }
 
   ml_close_all(false);              // close all memfiles, without deleting
-  stream_set_blocking(global_input_fd, true);  //normalize stream (#2598)
 
   mch_errmsg("Vim: Finished.\n");