Add non-destructive modes to pgindent

Add non-destructive modes to pgindent

This adds two modes of running pgindent, neither of which results in
any changes being made to the source code. The --show-diff option shows
what changes would have been made, and the --silent-diff option just
exits with a status of 2 if any changes would be made. The second of
these is intended for scripting use in places such as git hooks.

Along the way some code cleanup is done, and a --help option is also
added.

Reviewed by Tom Lane

Discussion: https://postgr.es/m/c9c9fa6d-6de6-48c2-4f8b-0fbeef026439@dunslane.net