MEDIUM: logs: remove the hostname, tag and pid part from the logheader

At the moment we have to call snprintf() for every log line just to
rebuild a constant. Thanks to sendmsg(), we send the message in 3 parts:
time-based header, proxy-specific hostname+log-tag+pid, session-specific
message.
diff --git a/include/proto/log.h b/include/proto/log.h
index 14dfc5b..e8b57d8 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -138,6 +138,11 @@
  */
 char *lf_port(char *dst, struct sockaddr *sockaddr, size_t size, struct logformat_node *node);
 
+/*
+ * Write hostname, log_tag and pid to the log string
+ */
+char *lf_host_tag_pid(char *dst, const char *hostname, const char *log_tag, int pid, size_t size);
+
 
 #endif /* _PROTO_LOG_H */