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]

file.c

@@ -4270,7 +4270,7 @@ rb_check_realpath_internal(VALUE basedir, VALUE path, enum rb_realpath_mode mode
 	}
     }
 
-    OBJ_INFECT(resolved, unresolved_path);
+    rb_obj_taint(resolved);
     RB_GC_GUARD(unresolved_path);
     RB_GC_GUARD(curdir);
     return resolved;

test/ruby/test_file.rb

@@ -298,7 +298,7 @@ class TestFile < Test::Unit::TestCase
       assert_predicate(File.realpath(base, dir), :tainted?)
       base.untaint
       dir.untaint
-      assert_not_predicate(File.realpath(base, dir), :tainted?)
+      assert_predicate(File.realpath(base, dir), :tainted?)
       assert_predicate(Dir.chdir(dir) {File.realpath(base)}, :tainted?)
     }
   end