client: rework command catenation

Use snprintf() instead of strcat() and don't try to parse commands
longer than 2048 characters to make it consistent with the chrony.conf
parser, avoid memory allocation, and not rely on the system ARG_MAX to
keep the length sane.

client.c

@@ -3245,37 +3245,30 @@ process_line(char *line)
 
 /* ================================================== */
 
+#define MAX_LINE_LENGTH 2048
+
 static int
 process_args(int argc, char **argv, int multi)
 {
-  int total_length, i, ret = 0;
-  char *line;
+  char line[MAX_LINE_LENGTH];
+  int i, l, ret = 0;
 
-  total_length = 0;
-  for(i=0; i<argc; i++) {
-    total_length += strlen(argv[i]) + 1;
-  }
-
-  line = (char *) Malloc((2 + total_length) * sizeof(char));
-
-  for (i = 0; i < argc; i++) {
-    line[0] = '\0';
-    if (multi) {
-      strcat(line, argv[i]);
-    } else {
-      for (; i < argc; i++) {
-        strcat(line, argv[i]);
-        if (i + 1 < argc)
-          strcat(line, " ");
-      }
+  for (i = l = 0; i < argc; i++) {
+    l += snprintf(line + l, sizeof (line) - l, "%s ", argv[i]);
+    if (l >= sizeof (line)) {
+      LOG(LOGS_ERR, "Command too long");
+      return 0;
     }
 
+    if (!multi && i + 1 < argc)
+      continue;
+
     ret = process_line(line);
     if (!ret || quit)
       break;
-  }
 
-  Free(line);
+    l = 0;
+  }
 
   return ret;
 }