OpenSystemsDevelopment / qpdf

This takes pages from the file in groups of n with default = 1. This
partially fixes the enhancement in issue #505 but doesn't implement
the entire suggestion.

Also fix a bug in checking consistency of length for stream data
providers. Length should not be checked or recorded if the provider
says it failed to generate the data.

For now, --allow-insecure allows creation of files with the owner
passwords empty or matching the user password.

When removing unreferenced resources, notice if a page (recursively)
contains a form XObject with unreferenced resources, and count any
such resources as referenced by the page.

Use the first of ASCII, PDFDocEncoding, or UTF-16 that is capable of
encoding the string.

Make some more methods in QPDFPageObjectHelper work with form
XObjects, provide forEach methods to walk through nested form
XObjects, possibly recursively. This should make it easier to work
with form XObjects from user code.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.