experimental: several links to the same port

(cherry picked from commit 0e368b1eaeedc0d3bf686832d825f5da7bc3a81b)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>

Conflicts:
	Singular/extra.cc
	Singular/links/ssiLink.cc (changes already ported)