Fix argument reduction of trig functions (see #155)

When subtracting multiples of 2pi, the multiple to subtract
was sometimes overestimated, leading to a negative result
where it was not expected (Taylor series where everything is
assumed to be positive).  Now fixed by making sure to
underestimate the multiple.  This still need work/thought.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4593 de43f980-851b-0410-b2f7-c40aca1f87e0