MINOR: log: move 'log' keyword parsing in dedicated function

Now, the function parse_logsrv should be used to parse a "log" line. This
function will update the list of loggers passed in argument. It can release all
log servers when "no log" line was parsed (by the caller) or it can parse "log
global" or "log <address> ... " lines. It takes care of checking the caller
context (global or not) to prohibit "log global" usage in the global section.
diff --git a/include/proto/log.h b/include/proto/log.h
index c92217c..05a7acc 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -82,6 +82,10 @@
  * 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, char **err);
+
+/* Parse "log" keyword and update the linked list. */
+int parse_logsrv(char **args, struct list *logsrvs, int do_del, char **err);
+
 /*
  * Displays the message on stderr with the date and pid. Overrides the quiet
  * mode during startup.