symbol-scan: verifies symbols-in-versions

This new script scans for all enums and #defines used by the curl/curl.h
and curl/multi.h headers. Then it reads all symbols mentioned in
symbols-in-vesions and make sure that there's no entries missing in
there. It then proceeds to verify that the entries that
symbols-in-vesions mentions but aren't found in the sources are truly
documented as removed.

This script is used in the new test case 1119