Bindings for c-ares, mostly autogenerated by rust-bindgen.

To create ffi.rs, do this:

bindgen -l cares -match ares -o src/ffi.rs c-ares/ares.h

... and then:

• add a few type definitions for OS types at the top of the file
  • Compare with the current version to see what's needed
• add unsafe to the function type declarations for ares_sock_state_cb, ares_callback, ares_host_callback, ares_nameinfo_callback, ares_sock_create_callback
  • Any non-trivial implementations of these callbacks will involve dereferencing raw pointer parameters
• remove ares_fds and ares_process
  • These both use fd_set.
  • Cursory search didn't find an existing Rust representation of this structure
  • Since the high-level API doesn't use either of these functions, it's simplest just to pretend they're not there

The script generate-ffi.sh automates this process.