REORG: stats: extract HTML related functions

Extract functions related to HTML stats webpage from stats.c into a new
module named stats-html. This allows to reduce stats.c to roughly half
of its original size.

Makefile

@@ -975,7 +975,7 @@ OBJS += src/mux_h2.o src/mux_fcgi.o src/mux_h1.o src/tcpcheck.o               \
         src/dynbuf.o src/wdt.o src/pipe.o src/init.o src/http_acl.o           \
         src/hpack-huff.o src/hpack-enc.o src/dict.o src/freq_ctr.o            \
         src/ebtree.o src/hash.o src/dgram.o src/version.o src/proto_rhttp.o   \
-        src/guid.o
+        src/guid.o src/stats-html.o
 
 ifneq ($(TRACE),)
   OBJS += src/calltrace.o