MEDIUM: log-format: Use standard HAProxy log system to report errors

The function log format emit its own error message using Alert(). This
patch replaces this behavior and uses the standard HAProxy error system
(with memprintf).

The benefits are:
 - cleaning the log system

 - the logformat can ignore the caller (actually the caller must set
   a flag designing the caller function).

 - Make the usage of the logformat function easy for future components.
diff --git a/include/proto/log.h b/include/proto/log.h
index f5a4f3f..2d4ca84 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -67,14 +67,14 @@
 /*
  * add to the logformat linked list
  */
-int add_to_logformat_list(char *start, char *end, int type, struct list *list_format);
+int add_to_logformat_list(char *start, char *end, int type, struct list *list_format, char **err);
 
 /*
  * Parse the log_format string and fill a linked list.
  * Variable name are preceded by % and composed by characters [a-zA-Z0-9]* : %varname
  * You can set arguments using { } : %{many arguments}varname
  */
-int parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int options, int cap);
+int parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int options, int cap, char **err);
 /*
  * Displays the message on stderr with the date and pid. Overrides the quiet
  * mode during startup.