UDP: Harmonize some function calls

Some calls use '(const u_char *)(up + 1)' or '(const void *)(up + 1)'.
Other calls use 'cp' (same value).
Update the calls to only use 'cp'.

Moreover:
Update some other casted 'up + 1' uses to 'cp'.
Update some pointers to packet bytes from 'void *' to 'u_char *'.