nitg: rename `common_ffi` group to `ffi`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix typo in doc of `MParameterType`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix typo in doc of `before_send`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: Move `common_ffi` to `ffi` and clean up typos

Renaming `common_ffi` was long overdue, it was named this way to avoid conflicts with its `nitc` equivalent.

Pull-Request: #752
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>