server: Deplete close on fork list on exit.

We empty the list in the command handler process by calling
close_listed_fds(), but the server process leaks the memory
allocated for the entries of the close on fork list.

This commit introduces deplete_close_on_fork_list() to empty the
list without closing any file descriptors. It is called from main()
to avoid the leak.

With the patch applied, valgrind --show-reachable=no no longer
complains about possibly lost blocks for the server process.