chg: allow assignment def=def (for "undefinig" list entries)
(cherry picked from commit a5281ab5104ed2cc0fc7a91c462921c5ebe96bbe)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>

Conflicts:
	Singular/ipassign.cc
fix: allow assignment def=def (for "undefinig" list entries)
(cherry picked from commit ee5be4b44ef3710c997a64552e4bf48a604dd4b3)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>
fix: allow assignment def=def (for "undefinig" list entries)
(cherry picked from commit 1b84dbde061938b92415371f1390ed6c3f31d328)

Signed-off-by: Andreas Steenpass <steenpass@mathematik.uni-kl.de>

Conflicts:
	Singular/table.h
Merge pull request #376 from steenpass/changes_from_master

Changes from master: allow assign of unused list elements
(needed in corner cases in libs)