Add support use system TimeZone

As explain in the issue :
fluent#593
Defined parameter : Time_Offset to "System"
This will use the system local timestamp.

Very usefull when the timezone depending on Summer/Winter

src/flb_parser.c

@@ -40,6 +40,7 @@
 #include <sys/stat.h>
 #include <limits.h>
 #include <string.h>
+#include <time.h>
 
 static inline uint32_t digits10(uint64_t v) {
     if (v < 10) return 1;
@@ -934,6 +935,8 @@ int flb_parser_tzone_offset(const char *str, int len, int *tmdiff)
     long min;
     const char *end;
     const char *p = str;
+    time_t t = time(NULL);
+    struct tm lt = {0};
 
     /* Check timezones */
     if (*p == 'Z') {
@@ -942,6 +945,15 @@ int flb_parser_tzone_offset(const char *str, int len, int *tmdiff)
         return 0;
     }
 
+    /* Check timezones */
+    if (*p == 'System') {
+        /* This is UTC, no changes required */
+
+        localtime_r(&t, &lt);
+        *tmdiff = lt.tm_gmtoff;
+        return 0;
+    }
+
     /* Unexpected timezone string */
     if (*p != '+' && *p != '-') {
         *tmdiff = 0;