A bunch of fixes mostly aimed at fixing the problems we have been

A bunch of fixes mostly aimed at fixing the problems we have been
having with leaking fds and with fatal errors occurring when the link
goes down.
Updated patchlevel.h to 2.4.2b2.
Moved open of /dev/ppp to generic_establish_ppp; we now close the
ppp_dev_fd in generic_disestablish_ppp rather than trying to use
PPPIOCDETACH.
*_send_config and *_recv_config now return 0 for success or -1 for
error, rather than calling fatal() when an error occurs.
Added a notifier for when we fork so plugins can close their fds in
the child.
Added a safe_fork() which does a fork and then closes stuff in the
child; the parent waits until the child has done that.
On detach, the parent rewrites the pid files rather than the child,
and the child waits for the parent to die.
Fixed some potential FILE * leaks.
Also moved auth_number() check into auth_check_options.