Explicitly allocate ResState on the call stack

Previously known as __res_state, res_state, res or even statp, the
ResState struct holds the context of a query in progress.

After a long series of preparatory patches, the thread-local storage
nonsense that's been plaguing this fine codebase for decades is finally
gone. Sayonara! ResState is now explicitly allocated on the stack at the
beginning of each query and passed down as a regular function argument.

To keep this patch reviewable, I threw in two compatibility typedefs for
the legacy names. We'll get rid of them in a separate mechanical patch.

Next steps:
 - Give ResState a regular constructor and, crucially, a real destructor
 - Move the definition to ResState.h, like it's a real class
 - Populate the nameservers immediately on construction
 - Switch to safe C++ data structures

Change-Id: I5c0b85da3ad9a55fa41dca6c87e57fe8f50c6abc