ares__swap_lists: make private and static

... since there's only one user, make it static within ares_process.c
ares__generate_new_id: moved to ares_query.c

... and ares__rc4 is turned into a local static function.
ares__bitncmp: use two underscores for private functions

It used a single one previously making it look like a public one
ares_get_datatype: removed unused function

it was also wrongly named as internal functions require two underscores