Thanks @Kabouik!

* 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

* Allow launching plugins with Alt + plugin's key

* Fix Alt key in filter/prompts modes

* Fix handling Alt key in nextsel()

In filter mode: run the associated plugin.
In prompt mode: just throw out the Alt+key input.
In nextsel(): differentiate Alt+key and Esc

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

Uses `NNN_FIFO`, minimal dependencies.

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

* Add a tabbed/xembed based file previewer plugin

This plugin is written in bash, because job control is not well
specified in POSIX sh (`jobs` can return anything).

We use `tabbed` [1] as a xembed [2] host, to have a single window
owning each previewer window.

Uses mpv, sxiv, zathura, and the nuke plugin.

[1]: http://tools.suckless.org/tabbed/
[2]: https://specifications.freedesktop.org/xembed-spec/xembed-spec-latest.html

* tabbed-preview: prevent focus steal with xdotool

* preview-tabbed: tabs->4 spaces

* preview-tabbed: add focus prevention timeout

* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

* Add nbak plugin to backup all nnn config

* nbak: check cd, quote env

* nbak: print backup file name

* nbak: add --show option, to show shell config

* nbak: fix shellcheck warning

'type' is POSIX complient, AND we check that we're actually running
bash, but shellcheck can't understand this...
Then '-o' is POSIX complient too, but shellcheck thinks it's "not well defined".

* nbak: variable renames, archive hierarchy changes

* nbak: fix variable expansion

* nbak: remove --show option

* nbak: call interactive bash/zsh to get fun/aliases

* Add nbak entry in plugins/README.md

* nbak: change archive hierarchy

* plugins/README.md: make nbak description shorter

* Added fzfz plugin

* Fixed shellcheck errors

* Fixed copy/paste error

* Added support for fzy, also renamed plugin since it's not specific to fzf anymore

* Refactored code

* Clean spillovers (jarun)

* Added fzfz plugin

* Fixed shellcheck errors

* Fixed copy/paste error

This plugin is based on .cbcp and does basically the reverse operation.
It copies system clipboard newline-separated file list to selection.

Some normal text was indent a little, which caused to to become a code