Fix how x is extracted in linf_min; increase tol.

Where by "increase tol," I mean that the linear program solver will
consider anything < tol = 1e-4 to be nonpositive. False positives in the
top row can lead to mistakenly thinking the solution set is unbounded,
while false positives in a pivot column can lead to precision problems
throughout the tableau.

Actually, it may be worthwhile to put in some kind of protection against
that sort of instability -- specifically, unitizing a very small pivot
element, or introducing small precision offsets by cancelling small
pivot column elements.