Fix wrapper macros so they don't try to dereference a function pointer

Fix wrapper macros so they don't try to dereference a function pointer
as a data value.  This doesn't work if you're using hpux cc, since
functions are aligned on 2-byte boundaries, but data has to be
accessed on 4-byte boundaries.  Accessing a function as a data value
isn't ANSI C portable anyway.  :-) Wrapper macros now take a function
pointer, and check to see if the function pointer is non-NULL, instead
of taking a (*funptr), and seeing if the function when treated as data
object is non-NULL.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@7803 dc483132-0cff-0310-8789-dd5450dbe970