Rework the calculation of the select timeout in io() to be based on absolute wake time

An absolute wake time is now calculated and updated by set_server_bits(),
TimerTimeout() and tclTimerTimeout().  This is then used to calculate the actual
select() timeout, instead of having those functions calculate a relative timeout
value themselves.  This significantly simplifies those functions, since the
underlying values tend to be recorded as absolute times.

It also allows us to disentangle tclTimeTimeout from TimerTimeout(), and move the
responsibility for calculating the QUEUE_SENDS wake time from TimerTimeout() to
set_server_bits() where it belongs.

The QUEUE_SENDS and CONNECT_DELAY wake up times will also now be correctly
calculated, based on the last sent time and connect time respectively.

include/tcl_bx.h

@@ -19,6 +19,7 @@ typedef struct {
 
 extern cmd_t C_msg[];
 extern cmd_t C_dcc[];
+void tclTimerTimeout(struct timeval *wake_time);
 int check_tcl_dcc (char *, char *, char *, int);
 
 void tcl_command (char *, char *, char *, char *);