Split out generic resolver functions.

adsbus/asyncaddrinfo.h

@@ -0,0 +1,8 @@
+#pragma once
+
+struct addrinfo;
+
+void asyncaddrinfo_init(size_t threads);
+void asyncaddrinfo_cleanup(void);
+int asyncaddrinfo_resolve(const char *node, const char *service, struct addrinfo *hints);
+int asyncaddrinfo_result(int fd, struct addrinfo **addrs);