Note that this is not compatible with 4 contexts.

* Add -a option to generate a temporary NNN_FIFO

* Add documentation for -a option

* plugins/README.md: promote the use of -a

This obsoletes the global FIFO unlink trick, so I remove mentions of it.
@jarun update:

Polish -a

This shows the "n³" logo as  icon in application menus, when desktop file is
installed (using `install-desktop` make target).

"n³" on a black knight chess piece/black horse head.

Install with `make install-desktop`
It now appears in application menus, search, etc. in XDG compatible
desktop environments (like GNOME, KDE, ...).

This reverts commit dbcbaa583c.

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT

* Add support for Haiku OS

* Adjust DISTFILES and dist rule in haiku

This reverts commit 278fcfdfc9.

- remove auto-stored session "@" on restore
- minor help update