Fixed minpre/minpost calculation wich neither worked on hour borders nor...

... on day borders.

Hi guys. I really like your VDR plugin and I have started to write a client side web frontend using the REST API (soon on Github). During testing I found this bug and the fix was pretty easy. Maybe I got the concept wrong so this might be subject to discussion. It follows a simple test case:

```c
#include <stdio.h>
#include <time.h>

int main(int argc, char **argv) {
	time_t estart = 86400*360*44 + 0*60*60;
	time_t estop = estart + 90*60;
	int start, stop;
	int minpre, minpost;
	struct tm *starttime, *stoptime;
	char buf[128];

	minpre = 5;
	minpost = 5;

	starttime = gmtime(&estart);
	start = starttime->tm_hour * 100 + starttime->tm_min - ((int)(minpre/60))*100 - minpre%60;

	strftime (buf, 128, "%Y-%m-%d %H:%M:%S %Z", starttime);
	printf("stime = %s\n", buf);
	
	stoptime = gmtime(&estop);
	stop = stoptime->tm_hour * 100 + stoptime->tm_min + ((int)(minpost/60))*100 + minpost%60;

	strftime (buf, 128, "%Y-%m-%d %H:%M:%S %Z", stoptime);
	printf("etime = %s\n", buf);

	printf("WRONG VERSION\n");
	printf("start = %04d\n", start);
	printf("stop = %04d\n", stop);

	printf("\n");
	printf("CORRECT VERSION\n");

	estart -= minpre*60;
	estop += minpost*60;
	starttime = gmtime(&estart);
	start = starttime->tm_hour * 100 + starttime->tm_min;

	stoptime = gmtime(&estop);
	stop = stoptime->tm_hour * 100 + stoptime->tm_min;

	strftime (buf, 128, "%Y-%m-%d %H:%M:%S %Z", stoptime);
	
	printf("start = %04d\n", start);
	printf("stop = %04d\n", stop);

	return 0;
}
```