Work around gcc warning about function pointers conversions.

Work around harmless (at least under POSIX systems where function pointers are
guaranteed to have the same representation as object pointers) but annoying
warnings given by gcc when converting between function and object pointers, e.g.

Source/DOH/fio.c: In function 'DohEncoding':
Source/DOH/fio.c:51: warning: ISO C forbids conversion of function pointer to object pointer type
Source/DOH/fio.c: In function 'encode':
Source/DOH/fio.c:75: warning: ISO C forbids conversion of object pointer to function pointer type
Source/DOH/base.c: In function 'DohCall':
Source/DOH/base.c:952: warning: ISO C forbids conversion of object pointer to function pointer type

Use an extra level of pointer indirection to avoid them.
Merge pull request #126 from vadz/warn-fix

Work around gcc warning about function pointers conversions.