Rename symlink_linux.go to symlink_posix.go

The code compiles just fine on Darwin, and 'posix' seems to fit both
cases better than having 2 copies that are the same implementation.
Merge pull request #13 from jameinel/support-darwin

Rename symlink_linux.go to symlink_posix.go

This allows it to compile on all platforms that have posix Symlink semantics (like in my case OS X).