BUG: Overflow in tan and tanh for large complex values

This is a fix for gh-2321.  I've imported the implementation for ctanh from FreeBSD's math library which handles large arguments correctly.  Which fixes this bug and the equivalent bug in np.tan.  

The problem here is that importing the function into npy_math_complex.c and adding a config check causes us to use the implementation from glibc on Linux, which also has this bug.  Although it seems to have been fixed in glibc last April (http://sourceware.org/bugzilla/show_bug.cgi?id=11521). 

I see several things that could be done here, the easiest is probably to use our version of ctanh unconditionally.  Although there are a multitude of ways to go about that, for instance should I remove the implementation from npy_math_complex.c and just place it directly in umath/funcs.inc, where the buggy version was?  Or should I just remove the config check and add a note about it somewhere? Or some other choice all together.

Some kind of test will also need to be added, but I thought I'd get some input on how to handle this before doing anything else.