OpenSystemsDevelopment / qpdf

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

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

This is needed for some compilers on some platforms, notably current
versions of clang.

StreamDataProvider::provideStreamData now has a rich enough API for it
to effectively proxy to pipeStreamData.

This was being saved until we had to break ABI.

Refactor removeUnreferencedResources to prepare for filtering form
XObjects.

This commit adds the preserve_eol flags but doesn't implement EOL
preservation yet.

Various PDF digital signing tools do not encrypt /Contents value in
signature dictionary. Adobe Acrobat Reader DC can handle a PDF with
the /Contents value not encrypted.

Write Contents in signature dictionary without encryption

Tests ensure that string /Contents are not handled specially when not
found in sig dicts.