dropbox: make dropboxd outlive `dropbox-cli start` (#277422)

buildFHSEnv {

  name = "dropbox";

  # The dropbox-cli command `dropbox start` starts the dropbox daemon in a

  # separate session, and wants the daemon to outlive the launcher.  Enabling

  # `--die-with-parent` defeats this and causes the daemon to exit when

  # dropbox-cli exits.

  dieWithParent = false;

  # dropbox-cli (i.e. nautilus-dropbox) needs the PID to confirm dropbox is running.

  # Dropbox's internal limit-to-one-instance check also relies on the PID.

  unsharePid = false;