extractCallingConvention checks function pointers

The extractCallingConvention function should check for any calling
convention attributes inside function pointer declarators.
dynamic foreign imports support stdcall

Dynamic foreign imports are generated by c2hs whenever C function
pointers need to be called. Function pointer declarations may contain
attributes which specify a calling convention to be used.

This patch Applies changes made in
a3ac24e1c7d3f2ec542364643f4346caba1a4915 to the case of "dynamic"
foreign imports.
Resolve FIXME comment with newly supplied argument
Merge branch 'calling_convention' of https://github.com/mikesteele81/c2hs into mikesteele81-calling_convention
Merge branch 'mikesteele81-calling_convention'