Previously, running pg_waldump with an invalid option (pg_waldump
--foo) would print the help output and exit successfully.  This was
because it tried to process the option letter '?' as a normal option,
but that letter is used by getopt() to report an invalid option.

To fix, process help and version options separately, like we do
everywhere else.  Also add a basic test suite for pg_waldump and run
the basic option handling tests, which would have caught this.