save/index/drecurse: correct handling for fifos and nonexistent paths.

save/index/drecurse: correct handling for fifos and nonexistent paths.

When indexing a fifo, you can try to open it (for security reasons) but it
has to be O_NDELAY just in case the fifo doesn't have anyone on the other
end; otherwise indexing can freeze.

In index.reduce_paths(), we weren't reporting ENOENT for reasons I can no
longer remember, but I think they must have been wrong.  Obviously if
someone specifies a nonexistent path on the command line, we should barf
rather than silently not back it up.

Add some unit tests to catch both cases.