All our targets support casts between function pointers and void *,
so we might as well use them.
This change was largely auto-generated, with the following prompts.
@getXXbyYY_r.c Remove the use of the `fct` union and replace it by
pointer casts.
Apply the same change to ether_* getnetgrent_r getnssent_r netname
publickey .
Do not use explicit `*` in function pointer calls. Replace
`(*((lookup_function) fct))` and similar with `((lookup_function) fct)`.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>