OpenSystemsDevelopment / qpdf

I thought /EFF was supposed to be used as a default for decrypting
embedded file streams, but actually it's supposed to be advice to a
conforming writer about handling new ones. This makes sense since the
findAttachmentStreams code, which is not actually needed, was never
right.

Add constructors that take a QPDF object so we can issue warnings and
create new indirect objects.

Avoid calling finish() multiple times on the pipeline passed to
pipeContentStreams. This commit also fixes a bug in which qpdf was not
exiting with the proper exit status if warnings found while splitting
pages; this was exposed by a test case that changed.

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.

Refactor QPDF_Stream to use stream filter classes to handle supported
stream filters as well.

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

External libraries for Windows are now built automatically in the
qpdf/external-libs repository and include openssl in addition to zlib
and jpeg. Use these, and update the Windows build to build with the
openssl crypto provider by default. We leave the native crypto
provider enabled in case there is a problem with openssl and also to
continue to exercise that code.

Specifically, if a stream had its stream data replaced and had
indirect /Filter or /DecodeParms, it would result in non-silent loss
of data and/or internal error.

There isn't really an issue with these files causing a real problem,
but malware and virus checkers trip on them, and the value to leaving
them in the test suite is too low to be worth the hassle.

Wildcard expansion is different in Windows from non-Windows and
sometimes requires special link options to work. Add tests that fail
if we link incorrectly.

This is suitable for use as a Lambda layer in AWS, inclusion in a
docker container, or other places where a minimal binary distribution
is desired.