Change -s behavior

Currently, one -s takes multiple paths (basically, it eats everything
until the next option, or --). I don't find it very intuitive or similar
to other programs. I suggest changing it to having to specify one -s for
each provider.

Before:

./tibeebuild -s prov1.so prov2.so -- trace

After

./tibeebuild -s prov1.so -s prov2.so trace