New timing infrastructure. There's a new function schedule_timer()

which pretty much any module can call to request a call-back in the
future. So terminal.c can do its own handling of blinking, visual
bells and deferred screen updates, without having to rely on
term_update() being called 50 times a second (fixes: pterm-timer);
and ssh.c and telnet.c both invoke a new module pinger.c which takes
care of sending keepalives, so they get sent uniformly in all front
ends (fixes: plink-keepalives, unix-keepalives).

[originally from svn r4906]
[this svn revision also touched putty-wishlist]

pinger.c

@@ -0,0 +1,71 @@
+/*
+ * pinger.c: centralised module that deals with sending TS_PING
+ * keepalives, to avoid replicating this code in multiple backends.
+ */
+
+#include "putty.h"
+
+struct pinger_tag {
+    int interval;
+    int pending;
+    long next;
+    Backend *back;
+    void *backhandle;
+};
+
+static void pinger_schedule(Pinger pinger);
+
+static void pinger_timer(void *ctx, long now)
+{
+    Pinger pinger = (Pinger)ctx;
+
+    if (pinger->pending && now - pinger->next >= 0) {
+	pinger->back->special(pinger->backhandle, TS_PING);
+	pinger->pending = FALSE;
+	pinger_schedule(pinger);
+    }
+}
+
+static void pinger_schedule(Pinger pinger)
+{
+    int next;
+
+    if (!pinger->interval) {
+	pinger->pending = FALSE;       /* cancel any pending ping */
+	return;
+    }
+
+    next = schedule_timer(pinger->interval * TICKSPERSEC,
+			  pinger_timer, pinger);
+    if (!pinger->pending || next < pinger->next) {
+	pinger->next = next;
+	pinger->pending = TRUE;
+    }
+}
+
+Pinger pinger_new(Config *cfg, Backend *back, void *backhandle)
+{
+    Pinger pinger = snew(struct pinger_tag);
+
+    pinger->interval = cfg->ping_interval;
+    pinger->pending = FALSE;
+    pinger->back = back;
+    pinger->backhandle = backhandle;
+    pinger_schedule(pinger);
+
+    return pinger;
+}
+
+void pinger_reconfig(Pinger pinger, Config *oldcfg, Config *newcfg)
+{
+    if (oldcfg->ping_interval != newcfg->ping_interval) {
+	pinger->interval = newcfg->ping_interval;
+	pinger_schedule(pinger);
+    }
+}
+
+void pinger_free(Pinger pinger)
+{
+    expire_timer_context(pinger);
+    sfree(pinger);
+}