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/if_cscope.c

@@ -820,7 +820,7 @@ err_closing:
     if (execl("/bin/sh", "sh", "-c", cmd, (char *)NULL) == -1)
       PERROR(_("cs_create_connection exec failed"));
 
-    stream_set_blocking(global_input_fd, true);  //normalize stream (#2598)
+    stream_set_blocking(input_global_fd(), true);  // normalize stream (#2598)
     exit(127);
   /* NOTREACHED */
   default:      /* parent. */