bus: Fix error handling for inotify_init()

• bus: Correct check for inotify_init() failure

  fd 0 is a valid fd - although if we get stdin as our inotify fd, something is weird somewhere. For the dbus-daemon, in practice this should never happen, because we use _dbus_ensure_standard_fds() to make sure stdin is already open. For unit tests, it could in theory be the case that stdin is closed.

• bus: Show the errno if inotify cannot be initialized

  This will hopefully help to indicate why.

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Prompted by #361 (closed).