This converts all slow syscalls in the Dir.empty? implementation
to release GVL.  We avoid unnecessarily GVL release and
reacquire for each slow call (opendir, readdir, closedir) and
instead only release and acquire the GVL once in the common
case.

Benchmark results show a small degradation in single-threaded
performance:
Execution time (sec)
name	trunk	built
dir_empty_p	0.689	0.758

Speedup ratio: compare with the result of `trunk' (greater is better)
name	built
dir_empty_p	0.909

* dir.c (rb_gc_for_fd_with_gvl): new function
  (nogvl_dir_empty_p): ditto
  (dir_s_empty_p): use new functions to release GVL
* benchmark/bm_dir_empty_p.rb: new benchmark
  [ruby-core:83071] [Feature #13958]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60111 b2dd03c8-39d4-4d8f-98ff-823fe69b080e