Fixes warning on CentOS 6:

  src/nnn.c:754: warning: ignoring return value of 'fwrite', declared with attribute warn_unused_result

The new location is ~/.config/nnn/.lastd

See #187. The behaviour has always remained so confusing. In my case a 0B file
opens in the GUI editor though I have NNN_USE_EDITOR set. We have a sensible
message in place to help users with what to do next.

Author: @bruce-hill

This may affect issue #227 but the defunct processes do not use any memory and exit when nnn exits. I guess that happens because of xdg-open using an intermediate script.

However, if we do not detach xdg-open, the child process is killed when the terminal is closed which is a severe problem.

This reverts commit 32dde3390a.