MINOR: diag: create cfgdiag module

This module is intended to serve as a placeholder for various
diagnostics executed after the configuration file has been fully loaded.
diff --git a/Makefile b/Makefile
index e8110bd..2ff6cd3 100644
--- a/Makefile
+++ b/Makefile
@@ -878,7 +878,7 @@
         src/ebistree.o src/auth.o src/wdt.o src/http_acl.o                     \
         src/hpack-enc.o src/hpack-huff.o src/ebtree.o src/base64.o             \
         src/hash.o src/dgram.o src/version.o src/fix.o src/mqtt.o src/dns.o    \
-        src/server_state.o src/proto_uxdg.o src/init.o
+        src/server_state.o src/proto_uxdg.o src/init.o src/cfgdiag.o
 
 ifneq ($(TRACE),)
 OBJS += src/calltrace.o