Add check mode support for non-python modules

Allows non-python modules to run in check mode.

If a module contains '#SUPPORTS_CHECK_MODE' and check mode is enabled, ansible will call the module even if it's not new-style python. The module will then receive the argument CHECKMODE=True.

Please note this pull request was developed at the same time as my next pull request 'Helper for bash modules'. There's a small reference in the doc to the helper module, so if the two pull requests are not merged at more or less the same time, please ask me to remove the reference. Thanks!