merge revision(s) r45521,r45523,r45551: [Backport #9699]

	win32.c: wchar conversion

	* win32/win32.c (rb_w32_wstr_to_mbstr, rb_w32_mbstr_to_wstr): make
  WCHAR/mb conversion functions public.
	* dln.c (dln_load): use wchar version to load a library in
	  non-ascii path on Windows.  based on the patch by Bugra Barin
	  <bugrabarin AT hotmail.com> in [ruby-core:61845].  [Bug #9699]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46450 b2dd03c8-39d4-4d8f-98ff-823fe69b080e