Support for individual session idle time locking. May be enabled by turning off

the lock-server option (it is on by default). When this is off, each session
locks when it has been idle for the lock-after-time setting. When on, the
entire server locks when ALL sessions have been idle for their individual
lock-after-time settings.

This replaces one global-only option (lock-after-time) with another
(lock-server), but the default behaviour is usually preferable so there don't
seem to be many alternatives.

Diff/idea largely from Thomas Adam, tweaked by me.

session.c

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <time.h>
 
 #include "tmux.h"
 
@@ -124,6 +125,7 @@ session_create(const char *name, const char *cmd, const char *cwd,
 	s = xmalloc(sizeof *s);
 	s->references = 0;
 	s->flags = 0;
+	s->activity = time(NULL);
 
 	if (gettimeofday(&s->tv, NULL) != 0)
 		fatal("gettimeofday failed");