Clarify documentation on encrypted files

Explicitly state how QPDF handles empty passwords when writing files.
Apparently some libraries treat the empty string as the owner password
as an instruction to generate a random password.
Export terminateParsing in the DLL

Windows fix: QPDFObject::ParserCallbacks::terminateParsing() was not
declared with QPDF_DLL.
Windows install: check DLL type

When copying dlls, make sure to only consider DLLs whose type matches
the type of what is loading them.