Handle LDAP operations asynchronously using nginx event architecture

A connection is opened using nginx framework and then fed to OpenLDAP using
ldap_init_fd() call with custom SockBuf IO handlers. When some credentials
need to be validated, ngx_http_auth_ldap_authenticate() is called multiple
times, returning NGX_AGAIN each time the process is waiting for the LDAP
server to reply.

(Note: This only an initial implementation and as such, it is of course
very buggy and limited. Further work is needed to make it useful.)

config

@@ -2,4 +2,4 @@ ngx_addon_name=ngx_http_auth_ldap_module
 HTTP_MODULES="$HTTP_MODULES ngx_http_auth_ldap_module"
 NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/ngx_http_auth_ldap_module.c"
 CORE_LIBS="$CORE_LIBS -lldap"
-CFLAGS="$CFLAGS -DLDAP_DEPRECATED"
+CFLAGS="$CFLAGS"