Add support for optional reboot delay

Slightly different take on issue #334 making it optional, possible to
enable per system.

    reboot-delay <0-60>           # default: 0 (disabled)

When enabled (non-zero), runs after filesystems have been unmounted,
the root filesystems has been remounted read-only, and sync(2) has
been called, twice.

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

src/finit.h

@@ -118,6 +118,7 @@ extern int    rescue;
 extern int    single;
 extern int    bootstrap;
 extern int    kerndebug;
+extern int    syncsec;
 extern char  *fstab;
 extern char  *sdown;
 extern char  *network;