Factor out ssh2_timer_update.

This is a preliminary refactoring for an upcoming change which will
need to affect every use of schedule_timer to wait for the next rekey:
those calls to schedule_timer are now centralised into a function that
does an organised piece of thinking about when the next timer should
be.

A side effect of this change is that the translation from
CONF_ssh_rekey_time to an actual tick count is now better proofed
against integer overflow (just in case the user entered a completely
silly value).

putty.h

@@ -27,6 +27,13 @@ typedef struct terminal_tag Terminal;
 #include "network.h"
 #include "misc.h"
 
+/*
+ * We express various time intervals in unsigned long minutes, but may need to
+ * clip some values so that the resulting number of ticks does not overflow an
+ * integer value.
+ */
+#define MAX_TICK_MINS	(INT_MAX / (60 * TICKSPERSEC))
+
 /*
  * Fingerprints of the PGP master keys that can be used to establish a trust
  * path between an executable and other files.