Delete dead code in res_comp.cpp and misc cleanups

No functionality changes, this is mostly meant to improve our code
coverage and cleanup our headers a bit.

 - delete dead functions res_ownok() and res_mailok()
 - Move prototypes of the remaining functions to the new header res_comp.h
 - Replace resolv_private.h with res_comp.h as needed
 - Drop the dependency on Bionic's <resolv.h> wherever possible
 - Eliminate the horrible maybe_XXX() macros
 - Make res_hnok() and res_dnok() return a proper bool instead of 0 and 1

Change-Id: Ic2e27753355d873925044a7561ccb78bd2c2d162