tmpdir.rb: permission of user given directory

	* lib/tmpdir.rb (Dir.mktmpdir): check if the permission of the
	  parent directory only when using the default temporary
	  directory, and no check against user given directory.  the
	  security is the user's responsibility in that case.
	  [ruby-core:91216] [Bug #15555]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66941 b2dd03c8-39d4-4d8f-98ff-823fe69b080e