Thanks @Kabouik!

* Added GPG encrypt and decrypt plugins

* Made description more descriptive

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

* preview-tui: simplify, add generic fifo_pager()

I've commented new filetype checks for now, as we need to discuss which ones
should be included by default, keeping in mind that this is supposed to be a
minimal, adaptable plugin.

* preview-tui: preview with man, tar, unzip by default

We leave it undefined on Solaris/Illumos because no such options exist,
so only use it if we have it.

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.

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.