NLv2: handle presolved independent linear subsystems

[jsiirola]

Fixes #3192.

Summary/Motivation:

This resolves the issue in the NL linear presolve for the case where the presolve identifies and removes an underdetermined linear independent subsystem.

Changes proposed in this PR:

• Set free variables in underdetermined linear independent subsystems to the feasible value closest to 0
• Add tests exercising this logic

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