Always mark the string returned by File.realpath as tainted

	This string can include elements that were not in either string
	passed to File.realpath, even if one of the strings is an
	absolute path, due to symlinks:

	```ruby
	Dir.mkdir('b') unless File.directory?('b')
	File.write('b/a', '') unless File.file?('b/a')
	File.symlink('b', 'c') unless File.symlink?('c')
	path = File.realpath('c/a'.untaint, Dir.pwd.untaint)
	path # "/home/testr/ruby/b/a"
	path.tainted? # should be true, as 'b' comes from file system
	```

	[Bug #15803]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67755 b2dd03c8-39d4-4d8f-98ff-823fe69b080e