Also disable atexit hooks in fatal_signal_fork().

Also disable atexit hooks in fatal_signal_fork().

This fixes a problem in ofp-discover, where it would daemonize itself
after setting an IP address on one of its interfaces.  The daemonize
function would call fatal_signal_fork() then exit(0), which would in turn
cause the netdev code to disable the interface and thereby remove the IP
address.

The bug that this fixes was introduced in commit 3cc1ae6a3, "Add ability
to run fatal signal hooks upon normal termination too."