[dhcp4relay] Migrate syslog() to SWSS_LOG macros for runtime log-level control (#110)

dhcp4relay used raw syslog() calls throughout, which meant every log
message regardless of level was unconditionally written to syslog with
no way to suppress verbose output at runtime. Under sustained DHCP
traffic the relay generates several hundred log lines per second,
saturating rsyslog rate limits and causing syslog to rotate every ~14
minutes, burying real errors.

This commit integrates swss::Logger so that log levels can be
controlled at runtime via swssloglevel without restarting the daemon.

Changes
-------
- main.cpp: add swss::Logger::linkToDbNative("dhcp4relay") to register
  with LOGLEVEL_DB at startup (default level: NOTICE).
- dhcp4relay.h: replace #include <syslog.h> with #include "logger.h".
  All .cpp files that include dhcp4relay.h (directly or transitively
  via dhcp4relay_mgr.h / dhcp4relay_stats.h) pick up logger.h through
  the existing include chain; no per-file change needed.
- dhcp4_sender.cpp: replace #include <syslog.h> with #include "logger.h".
- dhcp4relay.cpp, dhcp4relay_mgr.cpp, dhcp4relay_stats.cpp,
  dhcp4_sender.cpp, main.cpp: replace all 131 syslog() calls with the
  equivalent SWSS_LOG_* macros:
    LOG_ERR     -> SWSS_LOG_ERROR
    LOG_WARNING -> SWSS_LOG_WARN
    LOG_NOTICE  -> SWSS_LOG_NOTICE
    LOG_INFO    -> SWSS_LOG_INFO
    LOG_DEBUG   -> SWSS_LOG_DEBUG
    LOG_ALERT   -> SWSS_LOG_NOTICE  (signal_callback: receiving SIGTERM
                                     is normal operational behavior)
- Remove trailing \n from all format strings; SWSS_LOG adds its own.

Log level control after this change
------------------------------------
  swssloglevel -c dhcp4relay -l NOTICE   # production default
  swssloglevel -c dhcp4relay -l INFO     # verbose operation
  swssloglevel -c dhcp4relay -l DEBUG    # full per-packet tracing

No functional change to relay behavior.

Signed-off-by: Shivashankar CR <shivashankar.c.r@gmail.com>

Author: cshivashgit

dhcp4relay/src/dhcp4_sender.cpp

@@ -2,7 +2,7 @@
 
 #include <arpa/inet.h>
 #include <errno.h>
-#include <syslog.h>
+#include "logger.h"
 
 #include <cstring>
 
@@ -63,14 +63,14 @@ bool send_udp(int sock, uint8_t *buffer, struct sockaddr_in target, uint32_t len
         if (sendmsg(sock, &msg, 0) == -1) {
             char server_addr[INET_ADDRSTRLEN];
             inet_ntop(AF_INET, &(target.sin_addr), server_addr, INET_ADDRSTRLEN);
-            syslog(LOG_ERR, "sendmsg: Failed to send to target address: %s, error: %s\n", server_addr, strerror(errno));
+            SWSS_LOG_ERROR("sendmsg: Failed to send to target address: %s, error: %s", server_addr, strerror(errno));
             return false;
         }
     } else {
         if (sendto(sock, buffer, len, 0, (const struct sockaddr *)&target, sizeof(target)) == -1) {
             char server_addr[INET_ADDRSTRLEN];
             inet_ntop(AF_INET, &(target.sin_addr), server_addr, INET_ADDRSTRLEN);
-            syslog(LOG_ERR, "sendto: Failed to send to target address: %s, error: %s\n", server_addr, strerror(errno));
+            SWSS_LOG_ERROR("sendto: Failed to send to target address: %s, error: %s", server_addr, strerror(errno));
             return false;
         }
     }