MEDIUM: log: make http_sess_log use log_format

http_sess_log now use the logformat linked list to make the log
string, snprintf is not used for speed issue.

CLF mode also uses logformat.

NOTE: as of now, empty fields in CLF now are "" not "-" anymore.
diff --git a/include/proto/log.h b/include/proto/log.h
index 2cf114b..34d3813 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -113,6 +113,14 @@
  */
 int get_log_facility(const char *fac);
 
+/*
+ * Write a string in the log string
+ * Take cares of mandatory and quote options
+ *
+ * Return the adress of the \0 character, or NULL on error
+ */
+char *logformat_write_string(char *dst, char *src, size_t size, struct logformat_node *node);
+
 #endif /* _PROTO_LOG_H */
 
 /*