add typecheck_fn(type, function) to do type-checking of function
pointers.
Modified-by: NIngo Molnar <mingo@elte.hu>

(made it typeof() based, instead of typedef based.)
Signed-off-by: NChuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>