Rename struct __res_params to res_params

The double-underscore prefixes are a leftover from when this code lived
in bionic. Now we're slowly getting rid of them...

Also cleanup transitive includes a bit.

Test: atest resolv_integration_test resolv_unit_test
Change-Id: I65ed8aed273be4af37a982a167f364a1f33f9a36
18 files changed