OPTIM: halog: keep a fast path for the lines-count only

Using "halog -c" is still something quite common to perform on logs,
but unfortunately since the recent added controls, it was sensibly
slowed down due to the parsing of the accept date field.

Now we use a specific loop for the case where nothing is needed from
the input, and this sped up the line counting by 2.5x. A 2.4 GHz Xeon
now counts lines at a rate of 2 GB of logs per second.

contrib/halog/halog.c

@@ -631,6 +631,16 @@ int main(int argc, char **argv)
 	else if (filter & FILT_COUNT_ONLY)
 		line_filter = NULL;
 
+	if (!line_filter &&
+	    !(filter & (FILT_HTTP_ONLY|FILT_TIME_RESP|FILT_ERRORS_ONLY|FILT_HTTP_STATUS|FILT_QUEUE_ONLY|FILT_QUEUE_SRV_ONLY|FILT_TERM_CODE_NAME))) {
+		/* read the whole file at once first */
+		if (!filter_invert)
+			while (fgets2(stdin) != NULL)
+				lines_out++;
+
+		goto skip_filters;
+	}
+
 	while ((line = fgets2(stdin)) != NULL) {
 		linenum++;
 		time_field = NULL; accept_field = NULL;
@@ -788,7 +798,7 @@ int main(int argc, char **argv)
 			lines_out++; /* we're just counting lines */
 	}
 
-
+ skip_filters:
 	/*****************************************************
 	 * Here we've finished reading all input. Depending on the
 	 * filters, we may still have some analysis to run on the