* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

This shows the "n³" logo as  icon in application menus, when desktop file is
installed (using `install-desktop` make target).

Install with `make install-desktop`
It now appears in application menus, search, etc. in XDG compatible
desktop environments (like GNOME, KDE, ...).

Command: make O_PCRE=1 strip

make command:

make O_STATIC=1 O_NORL=1 strip

* Add support for Haiku OS

* Adjust DISTFILES and dist rule in haiku

* Don't pack sources (github does it better), sign sources

* Make CircleCI generate the same git archive as Github, return make dist

* Revert make clean too

* Upload signature to Github release

* Read Github token from an env variable

Use only the `strip` target fort stripping the binary.

$(CP) is not set by GNU Make
Seems like [1] is a full list of what GNU Make defines implicitly
[1] https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

Mistook CPPFLAGS ("C preprocessor flags") for CXXFLAGS ("C++ flags").

This reverts commit 0956b82d33.