basic/fd-util: sort the 'except' array in place

We need a sorted list of fds to skip over when closing. We would allocate a
copy of the passed array to do the sort. But all callers construct a temporary
array to pass to us, so it is pointless to copy it again.

close_all_fds/safe_fork_full/namespace_fork/fork_agent are changed to pass
a non-const int array. I checked all users, and all callers are fine with
the array being sorted.

The function was returning some number (sometimes 1, sometimes the extent
of the range passed over to close_range(), ???). Anyway, all callers only
check for error, so let's return 0 on success.