resolv: Move __res_context_hostalias into its own file and into libc

And reformat it to GNU style.  Remove the unecessary setbuf call.
Use __fgets_unlocked for PLT avoidance; no locking is required here.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>