[CLEANUP] move remaining stats sockets code to dumpstats

The remains of the stats socket code has nothing to do in proto_uxst
anymore and must move to dumpstats. The code is much cleaner and more
structured. It was also an opportunity to rename AN_REQ_UNIX_STATS
as AN_REQ_STATS_SOCK as the stats socket is no longer unix-specific
either.

The last item refering to stats in proto_uxst is the setting of the
task's nice value which should in fact come from the listener.
diff --git a/include/types/buffers.h b/include/types/buffers.h
index 0553116..b9836b2 100644
--- a/include/types/buffers.h
+++ b/include/types/buffers.h
@@ -113,7 +113,7 @@
 #define AN_REQ_HTTP_INNER       0x00000020  /* inner processing of HTTP request */
 #define AN_REQ_HTTP_TARPIT      0x00000040  /* wait for end of HTTP tarpit */
 #define AN_REQ_HTTP_BODY        0x00000080  /* inspect HTTP request body */
-#define AN_REQ_UNIX_STATS       0x00000100  /* process unix stats socket request */
+#define AN_REQ_STATS_SOCK       0x00000100  /* process stats socket request */
 
 #define AN_RTR_HTTP_HDR         0x00000200  /* inspect HTTP response headers */
 #define AN_REQ_PRST_RDP_COOKIE  0x00000400  /* persistence on rdp cookie */