dylib_scan_mode should account for private symbols

[fosterbrereton]

There should be a mode where private DLL symbols are not used for the purposes of ODRV detection. It doesn't mean those symbols cannot contribute to an ODRV, however. For example, if a DLL has its own implementations of new and delete and passes a dynamically allocated object across its DLL boundary, it would be (de)allocated with a mismatched pair of new and delete routines. Even so, such a filtering would help eliminate direct false positives for symbols that appear to conflict between components, but actually do not as one or more of them may be private.