* 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