Pass on `gtol` convergence criterion to linesearches that calculate `grad(f)`

[anriseth]

Morethuente, hagerzhang and strongwolfe all evaluate df.g! or df.fg!, and should thus report return any step that will give g(x+alpha*p) within tolerance from Optim, NLsolve etc.

[anriseth]

We should probably investigate whether the new step decreases f(x) as well. If not we must decrease the search interval.