Fix #20

set_relative_date was failing if the resulting date was in the previous
day.

src/include/ttail_init.h

  *- -#2s
  *- -#3Month
  *@param ttail_t ttail instance
- *@param char*[2] dates {min,max} both can be NULL
+ *@param char* date specifier
  *@param int c the date id to handle
  *@return -1 if error 0 else
  */

src/ttail_init.c

 				{
 					mod = 1;
 					value = abs(tm->tm_hour);
-					tm->tm_hour = 23 - (value % 24);
+					tm->tm_hour = 24 - (value % 24);
 					value /= 24;
 					value++;
 				}

tests/ttail_init_set_date_relative.c

 		{"#-10s", 10}, {"#-142sec", 142}, {"#-4096s", 4096},
 		{"#-1m", 60}, {"#-10min", 600},
 		{"#-1h", 3600}, {"#-10h", 36000},
+		{"#-2h", 7200}, {"#-12h", 43200},
 		{"#-1d", 3600*24}, {"#-2day", 3600*24*2},
 	};