Fix errors reported by Coverity

Thanks to Jiri Popelka from Red Hat for sending the output of a
Coverity run over qpdf.
Fix QPDFObjGen constructor implementation
Allow numeric range to be omitted qpdf --pages

Detect a missing page range and assume 1-z.
Add --show-npages option to qpdf
Adjust to-do list
Ignore broken DecodeParms for stream with no filters
Update for 4.2.0
Replace - with -- in documentation

This is a cosmetic change since both are accepted.
Update spelling checker word list
Prepare 4.2.0 release
Change API/ABI and withdraw 4.2.0

4.2.0 was binary incompatible in spite of there being no deletions or
changes to any public methods.  As such, we have to bump the ABI and
are fixing some API breakage while we're at it.

Previous 4.3.0 target is now 5.1.0.
Update doc for 5.0.0
Update valgrid suppressions
Prepare 5.0.0 release