integrate: Correctly handle manual, meijerg, risch flags

[normalhuman]

Only one of manual, meijerg, risch can be True, since True means "use only this method" (per the documentation ofintegrate). But this was not checked.

>>> integrate(1/x, x, risch=True, manual=True)
log(x)

Also, the option manual=True did not actually prevent other integration methods

>>> integrate(exp(x**2), x, manual=True)
sqrt(pi)*erfi(x)/2      # from meijerint

Both are corrected now: ValueError is raised if more than one True is given, and whenever one is True, the others are set to False.

[asmeurer]

The tests might have to be changed at some point, but the comments should make it clear enough what they are testing.