rbinstall.rb: default umask

	* tool/rbinstall.rb: revert r49841 and default umask to just
	  prohibit all from writing.  symlink(2) has no argument to set
	  permissions but is affected by umask.
	  [ruby-dev:49975] [Bug #13194]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57903 b2dd03c8-39d4-4d8f-98ff-823fe69b080e