Cleanly handle SIGTERM and SIGHUP

This allow using both NNN_OPTS and options with arguments from argv.

It's a little annoying that one has to press ^L followed by Esc to go back
to the normal view. We cannot reload the directory on Esc because that will
break the filter filtered view feature. So we take this route.

This is a trial patch to fix the typos

Signed-off-by: SZ Lin (林上智) <szlin@debian.org>

* 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

* preview-kitty: async version

The plugin now aborts slow file viewers.

* preview-kitty: use text mode preview for SVG

This allows to close preview windows opened just before exiting nnn.

* 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