If you have selected some files, you intend to work on them. Also, this prompt
expects <kbd>c</kbd> / <kbd>s</kbd> which should cancel the next prompt in the
delete workflow. Rest of the three workflows are non-fatal.

Benefits of removing this option:
- code reduction
- one less step in the relevant workflows (more than one prompt feels dragged)

This prompt shows in the following workflows:
- delete: users have explicitly requested to prefer sel over current
- batch rename: key <kbd>^R</kbd> renames a single file
- archive: in most of the cases the user would want to archive the selection
- create sym/hard link: not a frequently used workflow (in general)

* 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

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT

This reverts commit 278fcfdfc9.

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

This reverts commit cf388649b9.

- replace bools for internal non-persistent binary states with an uchar
- do not store path to session, derive from config path in tmp buffer
- program option to load session is not `-s`
- remove auto-stored session "@" on restore
- minor help update