Introduce time_offset() helper and replace open-coded versions in timer.c

This function takes a struct timeval and offsets it by a (potentially fractional)
number of seconds, given as a 'double'.

include/ircaux.h

@@ -74,6 +74,7 @@ double 	BX_time_diff 		(struct timeval, struct timeval);
 double time_since(const struct timeval *tv_from);
 double time_until(const struct timeval *tv_to);
 int time_cmp(const struct timeval *a, const struct timeval *b);
+struct timeval *time_offset(struct timeval *tv, double offset);
 char *	BX_plural 			(int);
 int	BX_time_to_next_minute 	(void);
 char *	BX_remove_trailing_spaces 	(char *);