ospfd: fix deferred shutdown handling

ospfd: fix deferred shutdown handling

The ospfd cleanup code is relatively complicated given the need to
appropriately handle the "max-metric router-lsa on-shutdown (5-100)"
command. When that command is configured and an OSPF instance is
unconfigured, the removal of the instance should be deferred to allow
other routers sufficient time to find alternate paths before the
local Router-LSAs are flushed. When ospfd is killed, however, deferred
shutdown shouldn't take place and all instances should be cleared
immediately.

This commit fixes a problem where ospf_deferred_shutdown_finish()
was prematurely exiting the daemon when no instances were left,
inadvertently preventing ospf_terminate() from clearing the ospfd
global variables. Additionally, the commit includes code refactoring
to enhance readability and maintainability.

Fixes #14855.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>