socket: fix getaddrinfo to handle international domain names (instead of calling stdlib's getaddrinfo)

core: remove EVENT_INTERNAL_AVAILABLE-related stuff. libevent-internal.h is not up to date with the latest libevent anyway.

the releases will continue to include it but it no longer useful to keep it in the repo as it's much easier to build now, setup.py will do that automatically.

 imp_find_dotted_module is based on code found in the modulegraph library.

setup.py: don't run ./configure if --libevent argument doesn't point to a libevent source directory.
I try to be compatible with the old behaviour here. Maybe setup.py should abort instead.