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.