Support fractional-second intervals on recurring timers

A fractional-second timeout was already supported for the initial timeout of a /TIMER, but
if the timer was recurring then the interval was rounded to a whole number of seconds for
the subsequent timeouts.

Change the type of interval from long to double to fix this.

include/struct.h

@@ -1043,7 +1043,7 @@ typedef struct	timerlist_stru
 	char	*command;
 	char	*subargs;
 	int	events;
-	time_t	interval;
+	double interval;
 	int	server;
 	int	window;
 	char	*whom;