Previously the code would create a dummy socket while resolving just to
have curl_multi_fdset() return something but the non-win32 version
doesn't do it this way and the creation and use of a socket that isn't
made with the common create-socket callback can be confusing to apps
using the multi_socket API etc.
This change removes the dummy socket and thus will cause
curl_multi_fdset() to return with maxfd == -1 more often.
Now use gai_strerror() to get proper error messages when getaddrinfo()
has failed. Detect the function in configure.
Code based on work and suggestions by Jeff Pohlmeyer and Guenter Knauf
asyn-ares.c and asyn-thread.c are two separate backends that implement
the same (internal) async resolver API for libcurl to use. Backend is
specified at build time.
The internal resolver API is defined in asyn.h for asynch resolvers.
