http_digest: Fixed memory leaks from commit 6f8d8131b1
contributors.sh: split lists using " and "

... and require the space after the filtering to make the filter able to
remove names.
THANKS-filter: added another Michał Górny version we've used
tool_strdup.c: include the tool strdup.h

... not the lib/ one that the tool no longer uses!
build: Fixed Visual Studio project file generation of strdup.[c|h]

As the curl command-line tool now includes it's own version of strdup(),
for platforms that don't have it, fixed up the git respository Visual
Studio project file generator to not include the version from lib in the
tool project files, rather than having both lib\strdup.[c|h] and
src\tool_strdup.[c|h] present.