VMS: ignore multiply defined symbols when linking programs

The Unix and Windows linkers appear to simply ignore if any symbol is
defined multiple times in different object files and libraries.

The VMS linker, on the other hand, warns about it, loud and clear.  It
will still create the executable, but does so screaming.  So we
complicate things by saving the linker output, look through all the
errors and warnings, and if they are only made up of %LINK-W-MULDEF,
we let it pass, otherwise we output the linker output and raise the
same exit code we got from the linker.

Reviewed-by: Emilia Käsper <emilia@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1789)