Note that this is not compatible with 4 contexts.

This supports using a power of 2 number of contexts like 2, 4 or 8.
Note that the number of contexts are not backward compatible. So if
user is having 8 contexts and plans to use 4, his sessions with > 1
contexts open may break.

We can have a make option to enable 8 contexts. 4 remains default.

This reverts commit 2232772db4.

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.

* 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 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

This fix previews window staying open after nnn exits, because they
inherited open FIFO (in write mode) from parent nnn process, and never
close it.