Dependency fixup

Two not very related commits:
* Fix a bug in the dependency cycle explanation. The code was referencing a non-existent local variable `p`, so it ended up calling the method `Kernel.p` with no arguments, which returned nil. This caused the error message generator to remove all plugins from the list of dependency-cycle-offender candidates.
* Fix a self-dependency in the solaris2 version of the DMI plugin. As explained in the commit message, that plugin is probably non-functional at the moment; it should be fixed by calling the default `collect_data` block from the solaris-specific one.

These two commits get ohai back to a working state (on my mac anyhow).