Move project version to file, add xS manual page

So far Go applications remain independent to handle Nix's inability
to easily combine them with the CMake part.

There is also no "install" target, because any packagers will want to
adjust installation paths manually, and there is no configure step.

xS/xS.adoc

@@ -0,0 +1,57 @@
+xS(1)
+=====
+:doctype: manpage
+:manmanual: xK Manual
+:mansource: xK {release-version}
+
+Name
+----
+xS - IRC daemon
+
+Synopsis
+--------
+*xS* [_OPTION_]...
+
+Description
+-----------
+*xS* is a basic IRC daemon for single-server networks, suitable for testing
+and private use.  When run without a configuration file, it will start listening
+on the standard port 6667 and the "any" address.
+
+Options
+-------
+*-debug*::
+	Do not daemonize, print more information on the standard error stream
+	to help debug various issues.
+
+*-systemd*::
+	Log using the format specified in *sd-daemon*(3).
+
+*-h*, *-help*::
+	Display a help message and exit.
+
+*-version*::
+	Output version information and exit.
+
+*-writedefaultcfg*::
+	Write a configuration file with defaults, show its path and exit.
++
+The file will be appropriately commented.
+
+Files
+-----
+*xS* follows the XDG Base Directory Specification.
+
+_~/.config/xS/xS.conf_::
+_/etc/xdg/xS/xS.conf_::
+	The daemon's configuration file.  Use the *-writedefaultcfg* option
+	to create a new one for editing.
+
+Reporting bugs
+--------------
+Use https://git.janouch.name/p/xK to report bugs, request features,
+or submit pull requests.
+
+See also
+--------
+*sd-daemon*(3)