Fix File.realpath reading relative symlinks.

Currently File.realpath fails reading relative symlinks.
It uses Dir.pwd to expand file path, while it should use current link path.
For example, when using homebrew:
`` irb(main):003:0*   File.realpath('/usr/local/opt/rbenv')
Errno::ENOENT: No such file or directory - /Users/andriy/dev/Cellar/rbenv/0.4.0
	from kernel/common/file19.rb:128:in `realpath'
	from (irb):3
	from kernel/common/block_environment.rb:75:in `call_on_instance'
	from kernel/common/eval.rb:75:in `eval'
	from kernel/common/kernel19.rb:42:in `loop'
	from kernel/bootstrap/proc.rb:22:in `call'
	from kernel/common/throw_catch19.rb:8:in `catch'
	from kernel/common/throw_catch.rb:10:in `register'
	from kernel/common/throw_catch19.rb:7:in `catch'
	from kernel/bootstrap/proc.rb:22:in `call'
	from kernel/common/throw_catch19.rb:8:in `catch'
	from kernel/common/throw_catch.rb:10:in `register'
	from kernel/common/throw_catch19.rb:7:in `catch'
	from kernel/common/codeloader.rb:212:in `require'
	from kernel/common/kernel.rb:638:in `gem_original_require (require)'
	from /Users/andriy/dev/rubinius/lib/rubygems/custom_require.rb:36:in `require'
	from kernel/loader.rb:681:in `irb'
	from kernel/loader.rb:817:in `main'irb(main):004:0> 
``