MEDIUM: http-htx/proxy: Use a global and centralized storage for HTTP error messages

All custom HTTP errors are now stored in a global tree. Proxies use a references
on these messages. The key used for errorfile directives is the file name as
specified in the configuration. For errorloc directives, a key is created using
the redirect code and the url. This means that the same custom error message is
now stored only once. It may be used in several proxies or for several status
code, it is only parsed and stored once.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 6ea96b3..232634d 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -427,7 +427,7 @@
 	char *check_path;			/* PATH environment to use for external agent checks */
 	char *expect_str;			/* http-check expected content : string or text version of the regex */
 	struct my_regex *expect_regex;		/* http-check expected content */
-	struct buffer errmsg[HTTP_ERR_SIZE];	/* default or customized error messages for known errors */
+	struct buffer *errmsg[HTTP_ERR_SIZE];	/* default or customized error messages for known errors */
 	int uuid;				/* universally unique proxy ID, used for SNMP */
 	unsigned int backlog;			/* force the frontend's listen backlog */
 	unsigned long bind_proc;		/* bitmask of processes using this proxy */