Refactor tty handling to use service backend enabling conditions etc.

Initial refactor of the tty implementation to use the service/run/task
general backend.  This enables all the features of services also for
ttys, except logging because it makes no sense.

Work in progress:

 - plugins/tty.c does not work anymore, could possibly be removed in
   favor of usinga (a new) condition instead (if-tty-exists)
 - fallback tty does not work anymore, should we remove it, or can we
   handle it as an optional built-in with (a new) condition?
 - @console does not work anymore, needs to generate N cloned services

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/finit.h

@@ -30,6 +30,7 @@
 #include <paths.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sysexits.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ -105,6 +106,7 @@ extern int    prevlevel;
 extern int    debug;
 extern int    rescue;
 extern int    single;
+extern int    bootstrap;
 extern char  *rcsd;
 extern char  *sdown;
 extern char  *network;