Replace dlopen with FFI.verify

[opalmer]

Unfortunately, dlopen does not always work which means we have to use FFI.verify instead. The Windows function GetNamedPipeHandleState for example ends up not being found in kernel32.dll. While this might be fixable, there are other reasons for moving away from dlopen:

• More consistent behavior across different Windows versions
• There's no longer a need to hard code constants in the Python source
• If we have to do something in C the background work necessary to allow this will already be complete
• Compile time warnings, such as mismatched types in input arguments, are more obvious