documentation changes

- removed known issue about modern mode as it should work now.
- removed note about callbacks being linux/x86 only.
- fixed typo: supported -> unsupported.
- remove note about [u]int{8,16,32,64} not being implemented.
- add documentation about uint, uchar, ulong, ullong.
- fix foreign-alloc's function signature.
- update info regarding lispworks's new foreign-funcall.
- add example for *darwin-framework-directories*.
- fixed define-foreign-library's syntax.
- comment out sentence about use-foreign-library setting the current
  foreign library as it doesn't do that yet.
- remove the (setf (callback ...)) example. this isn't allowed anymore.
- rename foreign-funcall and long-long to no-foreign-funcall and
  no-long-long.
- long-long is a standard C99 type, remove mention of GNU there.

- removed TODO item: there's already a tutorial.
- added TODO item: document defcfun's new capabilities