polyfit.m: Overhaul function (bug #57964). * polyfit.m: Rewrite documentation and add example code. Reshape all inputs into column vectors for Matlab compatibility. Validate number of points in x and y are the same. Force logical input N to be a row vector (no test previously). Validate that requested polynomial degree, N, can be achieved given size of data; issue a warning if it cannot and calculate a solution based on the lowest polynomial powers (x^0, x^1, ...). Test "isargout (2)" to avoid unnecessarily calculating complex outputs. Correctly size outputs s.R, s.C even when there is insufficient data. Correctly restrict degrees of freedom output to range >= 0. Add BIST tests for logical input N. Add BIST test for orientation of output. Add BIST test for insufficient data. Add BIST test for bug #57964. Add BIST tests for input validation.

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